libstdc++: Add missing _GLIBCXX_USE_WCHAR_T checks in testsuite

libstdc++: Add missing _GLIBCXX_USE_WCHAR_T checks in testsuite

These tests fail for a --disable-wchar_t build.

libstdc++-v3/ChangeLog:

	* testsuite/22_locale/conversions/buffer/1.cc: Check
	_GLIBCXX_USE_WCHAR_T.
	* testsuite/22_locale/conversions/buffer/3.cc: Likewise. Add
	test using char16_t.
	* testsuite/22_locale/conversions/string/1.cc: Check
	_GLIBCXX_USE_WCHAR_T.
	* testsuite/27_io/filesystem/path/generic/generic_string.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/modifiers/make_preferred.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/native/alloc.cc: Likewise.
	* testsuite/27_io/filesystem/path/native/string-char8_t.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/native/string.cc: Likewise.
	* testsuite/28_regex/algorithms/regex_match/extended/wstring_locale.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/generic/generic_string.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/native/alloc.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/native/string-char8_t.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/native/string.cc:
	Likewise.