Mention the tests in INSTALL.md

INSTALL.md

@@ -21,6 +21,8 @@ There are simple example programs in the examples directory, and you can view th
 
 https://examples.libsdl.org/SDL3
 
+More in-depth test programs are available in the tests directory and can be built by adding `-DSDL_TESTS=ON` to the CMake command line when building SDL.
+
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