[libc][obvious] only test FILE on working platforms

[michaelrj]

Summary

The main code for the FILE struct is only enabled on platforms that it
works on, but before this patch the tests were included unconditionally.

[sivachandra]

I thought a test was already skipped if it had missing deps. But, it looks like we skip a test only if it has skipped entrypoints: https://github.com/llvm/llvm-project/blob/main/libc/cmake/modules/LLVMLibCTestRules.cmake#L32.

While this change LGTM for now, I think a more robust way to skip tests would be by skipping if there are any missing deps. It can be done as a separate cleanup.

[lntue]

In D122363#3404232, @sivachandra wrote:

I thought a test was already skipped if it had missing deps. But, it looks like we skip a test only if it has skipped entrypoints: https://github.com/llvm/llvm-project/blob/main/libc/cmake/modules/LLVMLibCTestRules.cmake#L32.

While this change LGTM for now, I think a more robust way to skip tests would be by skipping if there are any missing deps. It can be done as a separate cleanup.

+1 on skipping on any missing dep in a separate cleanup.

libc/test/src/__support/File/CMakeLists.txt

+if(NOT (TARGET libc.src.__support.threads.mutex))
+  # Not all platforms have a mutex implementation. If mutex is unvailable,
+  # we just skip everything about files.
+  return()
+endif()
 
 add_libc_unittest(
   file_test
   SUITE
     libc_support_unittests
   SRCS
     file_test.cpp
   DEPENDS