libstdc++: Backport some std::format test improvements

This backports just the tests from r14-6469-g52de6aa1a85822 because the
regression wasn't present on this branch, but we can still use the
tests.

libstdc++-v3/ChangeLog:

	* testsuite/std/format/functions/format.cc: Check for expected
	output for char and bool arguments.
	* testsuite/std/format/string.cc: Check that 0 filling is
	rejected for character and string formats.

(cherry picked from commit 52de6aa1)