Updates to existing tests

This patch updates existing tests to validate the new value for the
__cpp_lib_char8_t feature test macros and to exercise u8path factory
function invocations with std::string, std::string_view, and interator
pair arguments.

2019-11-29  Tom Honermann  <tom@honermann.net>

	Updates to existing tests
	* testsuite/experimental/feat-char8_t.cc: Updated the expected
	__cpp_lib_char8_t feature test macro value.
	* testsuite/27_io/filesystem/path/factory/u8path.cc: Added testing of
	u8path invocation with std::string, std::string_view, and iterators
	thereof.
	* testsuite/experimental/filesystem/path/factory/u8path.cc: Added
	testing of u8path invocation with std::string, std::string_view, and
	iterators thereof.

From-SVN: r278857