Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```

I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
   if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
        find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
    else
        find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
    fi
done
```

Fixes https://github.com/libsdl-org/SDL/issues/6575

include/SDL3/SDL_test_harness.h

@@ -0,0 +1,134 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/**
+ *  \file SDL_test_harness.h
+ *
+ *  Include file for SDL test framework.
+ *
+ *  This code is a part of the SDL test library, not the main SDL library.
+ */
+
+/*
+  Defines types for test case definitions and the test execution harness API.
+
+  Based on original GSOC code by Markus Kauppila <markus.kauppila@gmail.com>
+*/
+
+#ifndef SDL_test_h_arness_h
+#define SDL_test_h_arness_h
+
+#include <SDL3/begin_code.h>
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* ! Definitions for test case structures */
+#define TEST_ENABLED  1
+#define TEST_DISABLED 0
+
+/* ! Definition of all the possible test return values of the test case method */
+#define TEST_ABORTED        -1
+#define TEST_STARTED         0
+#define TEST_COMPLETED       1
+#define TEST_SKIPPED         2
+
+/* ! Definition of all the possible test results for the harness */
+#define TEST_RESULT_PASSED              0
+#define TEST_RESULT_FAILED              1
+#define TEST_RESULT_NO_ASSERT           2
+#define TEST_RESULT_SKIPPED             3
+#define TEST_RESULT_SETUP_FAILURE       4
+
+/* !< Function pointer to a test case setup function (run before every test) */
+typedef void (*SDLTest_TestCaseSetUpFp)(void *arg);
+
+/* !< Function pointer to a test case function */
+typedef int (*SDLTest_TestCaseFp)(void *arg);
+
+/* !< Function pointer to a test case teardown function (run after every test) */
+typedef void  (*SDLTest_TestCaseTearDownFp)(void *arg);
+
+/**
+ * Holds information about a single test case.
+ */
+typedef struct SDLTest_TestCaseReference {
+    /* !< Func2Stress */
+    SDLTest_TestCaseFp testCase;
+    /* !< Short name (or function name) "Func2Stress" */
+    const char *name;
+    /* !< Long name or full description "This test pushes func2() to the limit." */
+    const char *description;
+    /* !< Set to TEST_ENABLED or TEST_DISABLED (test won't be run) */
+    int enabled;
+} SDLTest_TestCaseReference;
+
+/**
+ * Holds information about a test suite (multiple test cases).
+ */
+typedef struct SDLTest_TestSuiteReference {
+    /* !< "PlatformSuite" */
+    const char *name;
+    /* !< The function that is run before each test. NULL skips. */
+    SDLTest_TestCaseSetUpFp testSetUp;
+    /* !< The test cases that are run as part of the suite. Last item should be NULL. */
+    const SDLTest_TestCaseReference **testCases;
+    /* !< The function that is run after each test. NULL skips. */
+    SDLTest_TestCaseTearDownFp testTearDown;
+} SDLTest_TestSuiteReference;
+
+
+/**
+ * \brief Generates a random run seed string for the harness. The generated seed will contain alphanumeric characters (0-9A-Z).
+ *
+ * Note: The returned string needs to be deallocated by the caller.
+ *
+ * \param length The length of the seed string to generate
+ *
+ * \returns the generated seed string
+ */
+char *SDLTest_GenerateRunSeed(const int length);
+
+/**
+ * \brief Execute a test suite using the given run seed and execution key.
+ *
+ * \param testSuites Suites containing the test case.
+ * \param userRunSeed Custom run seed provided by user, or NULL to autogenerate one.
+ * \param userExecKey Custom execution key provided by user, or 0 to autogenerate one.
+ * \param filter Filter specification. NULL disables. Case sensitive.
+ * \param testIterations Number of iterations to run each test case.
+ *
+ * \returns the test run result: 0 when all tests passed, 1 if any tests failed.
+ */
+int SDLTest_RunSuites(SDLTest_TestSuiteReference *testSuites[], const char *userRunSeed, Uint64 userExecKey, const char *filter, int testIterations);
+
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_test_h_arness_h */
+
+/* vi: set ts=4 sw=4 expandtab: */