Fix filesystem::path tests

Fix filesystem::path tests

The root_path.cc test had some debugging macros left in accidentally, so
didn't FAIL correctly if an assertion failed.

The string-char8_t.cc tests didn't compile on Windows.

	* testsuite/27_io/filesystem/path/decompose/root_path.cc: Remove
	macros accidentally left in.
	* testsuite/27_io/filesystem/path/native/string-char8_t.cc: Remove
	unnecessary -lstdc++fs option. Fix test for mingw.
	* testsuite/experimental/filesystem/path/native/string-char8_t.cc:
	Fix test for mingw.

From-SVN: r270685