Improve `P0218R1_filesystem` test reliability and temp filename generation

[StephanTLavavej]

Making an exception to our usual rule of "we don't spend a lot of effort on cleaning up tests that are well-behaved, because it's low reward and risks disrupting what they were trying to test". I had to investigate why P0218R1_filesystem was behaving slightly flaky, and I ended up making a bunch of targeted improvements.

• Centralize the definition of temp_file_name().
  • In #3337, I pushed a commit to copy tests/tr1/include/temp_file_name.h to tests/std/include/temp_file_name.hpp, because tests/std can't use tests/tr1/include. What I forgot is that tests/tr1 can use tests/std/include, in both the GitHub and MSVC-internal test harnesses. Let's deduplicate these files now.
  • Include <temp_file_name.hpp> with angle brackets because it's outside the current directory (this is our usual convention, with only a few exceptions that I'll clean up later).
• Improve temp_file_name(): "msvc_stl_" prefix, 128 bits.
  • The ".tmp" extension is already clear, so let's change the prefix to make the source of these files obvious.
  • In #2210, I chose 256 bits of entropy without careful consideration, and it was way too much. 128 bits is plenty; see Wikipedia's birthday problem article for a probability table.
• Conversely, increase get_test_directory_subname() from 16 hexits / 64 bits to 32 hexits / 128 bits.
• Rename part 1: get_test_directory => get_experimental_test_directory
• Rename part 2: get_new_test_directory => get_test_directory
  • Standard <filesystem> should be the default assumption.
• Simplify get_test_directory_subname() by templating it.
  • We no longer need <cstring> for strlen().
• Make test_temp_directory resistant to misuse.
  • Mark it as a [[nodiscard]] guard type and make it noncopyable.
• test_temp_directory's ctor/dtor can use local error_code ec;.
  • Nobody needed to access this as a public data member after construction.
• Consistently name test_temp_directory, part 1: recursiveTests => tempDir
• Consistently name test_temp_directory, part 2: followSymlinkTests => tempDir
• Use consistent names for test directory bases.
  • "recursive_directory_iterator-specific": Separate with a dash, instead of an accursed space.
  • "recursive_directory_iterator-VSO-649431": Separate with a dash, instead of an underscore.
  • "status": We use the name of the filesystem function we're testing, not our outer test function.
  • "create_directories-and-remove_all": Separate with dashes, don't abbreviate.
• Directly pass paths instead of .native().
  • These calls to create_directories(), create_directory(), and exists() were totally inconsistent for no reason. We don't need to construct temporary identical paths.
• Improve testing of "Standard rename where target doesn't exist".
  • We should call the ec form (since we're not catching exceptions), verify that ec is good, and verify that the source file no longer exists.
• Skip affected rename() tests for _MSVC_INTERNAL_TESTING.

[StephanTLavavej]

I'm speculatively mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.