These tests check that various macros, constants and typedefs in <stdint.h> and <stddef.h> are correct.
There are two essentially identical source files, one in C and one in C++. Certain types are built into C++ but not into C, so we need to test both. Additionally, writing very precise tests comparing values or types is considerably easier with C++ machinery like decltype and templated type traits.
Since wchar_t in particular is so finicky, it is separately tested in source files that only include <wchar.h>. In the past there were bugs where the wchar_t header was broken in a way that was masked by the presence of <stdint.h>, which is very often the case.
This level of detail in tests is valuable in part because compilers (GCC and Clang) differ in what predefined macros they create. For instance, sufficiently new GCCs define __WCHAR_MIN__ as a builtin, but Clang does not.