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.