Extremely slow tests rpl.rpl_non_direct_mixed_mixing_engines and rpl.rpl_stm_mixing_engines

The replication tests rpl.rpl_non_direct_mixed_mixing_engines and rpl.rpl_stm_mixing_engines execute very slowly: over 6 minutes (240 seconds) on my system with an optimized debug build, and almost 8 minutes (480 seconds) on the mandatory staging builder that runs MemorySanitizer with --big-test.

I think that the tests must be split or cleaned up so that they spend less time waiting for something and more time executing interesting steps. Because the tests are marked as big, they run very seldomly, and despite that, they fail pretty often.

As a temporary work around, I will disable the tests on the MSAN builder, hoping to reduce the per-push test run time by a couple of minutes.