lib/drmtest: restore in_fixture assert

I didn't really want to revert those, too ...

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

lib/drmtest.c

@@ -654,6 +654,8 @@ static enum {
 
 bool __igt_fixture(void)
 {
+	assert(!in_fixture);
+
 	if (igt_only_list_subtests())
 		return false;
 
@@ -666,11 +668,15 @@ bool __igt_fixture(void)
 
 void __igt_fixture_complete(void)
 {
+	assert(in_fixture);
+
 	in_fixture = false;
 }
 
 void __igt_fixture_end(void)
 {
+	assert(in_fixture);
+
 	in_fixture = false;
 	longjmp(igt_subtest_jmpbuf, 1);
 }