lib: Make igt_skip noreturn

This is a remnant from the subtest code before we had proper
fixture blocks. With those there's no no reason to have igt_skip
calls outside of fixtures or subtests, so we can assert this and
hence always jump out.

Suggested by Thomas Wood since static analyzers got confused about
this.

To check for fallout I've run all tests as non-root so that they'll
all skip. Only very little fallout resulted.

Cc: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

lib/drmtest.c

@@ -1027,8 +1027,8 @@ void igt_skip(const char *f, ...)
 		exit_subtest("SKIP");
 	} else if (test_with_subtests) {
 		skip_subtests_henceforth = SKIP;
-		if (in_fixture)
-			__igt_fixture_end();
+		assert(in_fixture);
+		__igt_fixture_end();
 	} else {
 		exit(77);
 	}