testsuite: Uniquify test names [PR 98795]

testsuite: Uniquify test names [PR 98795]

Header unit names come from the path the preprocessor determines, and
thus can be absolute.  This tweaks the testsuite to elide that
absoluteness when embedded in a CMI name.  We were also not
distinguishing link and execute tests by the $std flags, so append
them when necessary.

	PR testsuite/98795
	gcc/testsuite/
	* g++.dg/modules/modules.exp (module_cmi_p): Avoid
	embedded absolute paths.
	(module_do_it): Append $std to test name.