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_random.h

@@ -0,0 +1,115 @@
+/*
+  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_random.h
+ *
+ *  Include file for SDL test framework.
+ *
+ *  This code is a part of the SDL test library, not the main SDL library.
+ */
+
+/*
+
+ A "32-bit Multiply with carry random number generator. Very fast.
+ Includes a list of recommended multipliers.
+
+ multiply-with-carry generator: x(n) = a*x(n-1) + carry mod 2^32.
+ period: (a*2^31)-1
+
+*/
+
+#ifndef SDL_test_random_h_
+#define SDL_test_random_h_
+
+#include <SDL3/begin_code.h>
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* --- Definitions */
+
+/*
+ * Macros that return a random number in a specific format.
+ */
+#define SDLTest_RandomInt(c)        ((int)SDLTest_Random(c))
+
+/*
+ * Context structure for the random number generator state.
+ */
+  typedef struct {
+    unsigned int a;
+    unsigned int x;
+    unsigned int c;
+    unsigned int ah;
+    unsigned int al;
+  } SDLTest_RandomContext;
+
+
+/* --- Function prototypes */
+
+/**
+ *  \brief Initialize random number generator with two integers.
+ *
+ *  Note: The random sequence of numbers returned by ...Random() is the
+ *  same for the same two integers and has a period of 2^31.
+ *
+ *  \param rndContext     pointer to context structure
+ *  \param xi         integer that defines the random sequence
+ *  \param ci         integer that defines the random sequence
+ *
+ */
+ void SDLTest_RandomInit(SDLTest_RandomContext * rndContext, unsigned int xi,
+                  unsigned int ci);
+
+/**
+ *  \brief Initialize random number generator based on current system time.
+ *
+ *  \param rndContext     pointer to context structure
+ *
+ */
+ void SDLTest_RandomInitTime(SDLTest_RandomContext *rndContext);
+
+
+/**
+ *  \brief Initialize random number generator based on current system time.
+ *
+ *  Note: ...RandomInit() or ...RandomInitTime() must have been called
+ *  before using this function.
+ *
+ *  \param rndContext     pointer to context structure
+ *
+ *  \returns a random number (32bit unsigned integer)
+ *
+ */
+ unsigned int SDLTest_Random(SDLTest_RandomContext *rndContext);
+
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_test_random_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */