Coverage merge failures cause test_process_slow stderr check to fail

When tor's coverage CI fails to merge coverage records, it logs to stderr. This causes the process stderr checks to fail.

slow/process/callbacks: profiling:/home/travis/build/teor2345/tor/src/trunnel/src_trunnel_libor_trunnel_testing_a-netinfo.gcda:Merge mismatch for function 47
  FAIL src/test/test_process_slow.c:241: assert(smartlist_len(process_data->stderr_data) OP_EQ 3): 4 vs 3
  [callbacks FAILED]

https://travis-ci.org/teor2345/tor/jobs/477561625#L5661

We could skip this test when coverage is active? Or even better, we could allow 3 or 4 stderr lines when coverage is active?

This is a critical bug, because it's a CI failure. But it doesn't fail all the time. Maybe we should turn off CI caching on coverage builds, or exclude coverage records from the CI cache?