tests: make test-verify-repo-operations.py not run by default test-verify-repo-operations.py currently starts way too late and extends the running time with -j50 on my machine from around 3:48 min to 6:30 min. We could of course make it run earlier, but the test case seems unlikely to find bugs not covered by other tests, so let's mark it "slow" instead. I think this type of test is better suited to running separately in a long-running job.