libstdc++: Check conversion from filesystem::path to wide strings [PR95048]

The testcase added for this bug only checks conversion from wide strings
on construction, but the fix also covered conversion to wide stings via
path::wstring(). Add checks for that, and u16string() and u32string().

libstdc++-v3/ChangeLog:

	PR libstdc++/95048
	* testsuite/27_io/filesystem/path/construct/95048.cc: Check
	conversions to wide strings.
	* testsuite/experimental/filesystem/path/construct/95048.cc:
	Likewise.