test_ukify: propagate failure

Oops. This explains why the tests were "passing" in CI even
though a direct pytest invocation would fail.

src/ukify/test/test_ukify.py

@@ -495,4 +495,4 @@ def test_pcr_signing2(kernel_initrd, tmpdir):
     assert len(sig['sha1']) == 6   # six items for six phases paths
 
 if __name__ == '__main__':
-    pytest.main([__file__, '-v'])
+    sys.exit(pytest.main([__file__, '-v']))