tests/gem_concurrent_blt: Adjust subtest naming

Our tooling doesn't cope with () in the testnames (piglit becomes all
confused apparently) and the naming convention says to use "blt" and
"render".

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88220
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88349
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

tests/gem_mmap_wc.c

@@ -429,7 +429,7 @@ igt_main
 	igt_fixture
 		fd = drm_open_any();
 
-	igt_subtest("invalid flags")
+	igt_subtest("invalid-flags")
 		test_invalid_flags(fd);
 	igt_subtest("copy")
 		test_copy(fd);