Add a test to validate hostfxr C API

[am11]

Add a test for #109763 and align the style as headers in src/native/minipal/*.h.

[dotnet-policy-service]

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[am11]

e.g. without the struct keyword, it failed to build tests

  In file included from /Users/am11/projects/runtime5/src/native/corehost/test/mockhostfxr/test_c_api.c:8:
  /Users/am11/projects/runtime5/src/native/corehost/hostfxr.h:419:20: error: must use 'struct' tag to refer to type 'hostfxr_initialize_parameters'
    419 |     /*opt*/ const  hostfxr_initialize_parameters* parameters,
        |                    ^
        |                    struct 
  1 error generated.
  make[2]: *** [test/mockhostfxr/CMakeFiles/mockhostfxr_5_0.dir/test_c_api.c.o] Error 1
  make[2]: *** Waiting for unfinished jobs....
  [ 73%] Building CXX object fxr/standalone/CMakeFiles/hostfxr.dir/__/corehost_init.cpp.o
  make[1]: *** [test/mockhostfxr/CMakeFiles/mockhostfxr_5_0.dir/all] Error 2

[am11]

Had to move it after the struct hostfxr_dotnet_environment_framework_info is defined, to fix:

  In file included from /Users/am11/projects/runtime5/src/native/corehost/test/mockhostfxr/test_c_api.c:8:
  /Users/am11/projects/runtime5/src/native/corehost/hostfxr.h:91:18: error: declaration of 'struct hostfxr_dotnet_environment_info' will not be visible outside of this function [-Werror,-Wvisibility]
     91 |     const struct hostfxr_dotnet_environment_info* info,
        |                  ^
  1 error generated.
  make[2]: *** [test/mockhostfxr/CMakeFiles/mockhostfxr_2_2.dir/test_c_api.c.o] Error 1
  make[2]: *** Waiting for unfinished jobs....

[am11]

Backport together with #110060, since it fixes another ordering issue related to C vs. C++.

[elinor-fung]

Thank you!