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.

libstdc++-v3/ChangeLog:

2019-09-15  Tom Honermann  <tom@honermann.net>

      * libstdc++-v3/testsuite/experimental/feat-char8_t.cc: Updated the
        expected __cpp_lib_char8_t feature test macro value.
      * libstdc++-v3/testsuite/27_io/filesystem/path/factory/u8path.cc:
        Added testing of u8path invocation with std::string,
        std::string_view, and iterators thereof.
      * 
libstdc++-v3/testsuite/experimental/filesystem/path/factory/u8path.cc:
        Added testing of u8path invocation with std::string,
        std::string_view, and iterators thereof.

Tom.