depcomp tests: avoid spurious failures with 'dashXmstdout' depmode

depcomp tests: avoid spurious failures with 'dashXmstdout' depmode

* tests/gen-testsuite-part: The 'dashXmstdout' depmode cannot be
truly forced on gcc (like we were trying to do, causing spurious
failures in the process); so don't do that.  This change shouldn't
actually reduce coverage anyway, since, as the comments in
'lib/depcomp' say, the 'dashXmstdout' depmode is just like
the 'dashmstdout' one, only with a different argument for the
compiler.