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_assert.h

@@ -0,0 +1,105 @@
+/*
+  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_assert.h
+ *
+ *  Include file for SDL test framework.
+ *
+ *  This code is a part of the SDL test library, not the main SDL library.
+ */
+
+/*
+ *
+ * Assert API for test code and test cases
+ *
+ */
+
+#ifndef SDL_test_assert_h_
+#define SDL_test_assert_h_
+
+#include <SDL3/begin_code.h>
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * \brief Fails the assert.
+ */
+#define ASSERT_FAIL     0
+
+/**
+ * \brief Passes the assert.
+ */
+#define ASSERT_PASS     1
+
+/**
+ * \brief Assert that logs and break execution flow on failures.
+ *
+ * \param assertCondition Evaluated condition or variable to assert; fail (==0) or pass (!=0).
+ * \param assertDescription Message to log with the assert describing it.
+ */
+void SDLTest_Assert(int assertCondition, SDL_PRINTF_FORMAT_STRING const char *assertDescription, ...) SDL_PRINTF_VARARG_FUNC(2);
+
+/**
+ * \brief Assert for test cases that logs but does not break execution flow on failures. Updates assertion counters.
+ *
+ * \param assertCondition Evaluated condition or variable to assert; fail (==0) or pass (!=0).
+ * \param assertDescription Message to log with the assert describing it.
+ *
+ * \returns the assertCondition so it can be used to externally to break execution flow if desired.
+ */
+int SDLTest_AssertCheck(int assertCondition, SDL_PRINTF_FORMAT_STRING const char *assertDescription, ...) SDL_PRINTF_VARARG_FUNC(2);
+
+/**
+ * \brief Explicitly pass without checking an assertion condition. Updates assertion counter.
+ *
+ * \param assertDescription Message to log with the assert describing it.
+ */
+void SDLTest_AssertPass(SDL_PRINTF_FORMAT_STRING const char *assertDescription, ...) SDL_PRINTF_VARARG_FUNC(1);
+
+/**
+ * \brief Resets the assert summary counters to zero.
+ */
+void SDLTest_ResetAssertSummary(void);
+
+/**
+ * \brief Logs summary of all assertions (total, pass, fail) since last reset as INFO or ERROR.
+ */
+void SDLTest_LogAssertSummary(void);
+
+
+/**
+ * \brief Converts the current assert summary state to a test result.
+ *
+ * \returns TEST_RESULT_PASSED, TEST_RESULT_FAILED, or TEST_RESULT_NO_ASSERT
+ */
+int SDLTest_AssertSummaryToTestResult(void);
+
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_test_assert_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */