tests: Fix race in test_backwards_changelog_compatibilty

This test was suffering from a race: after calling

  mfsmetalogger start

we can't be sure if changelog_ml.mfs.1 and changelog_ml.mfs.2 are just
renamed from previous names or renamed and overwritten by downloading
two last changelogs from the master server.

Change-Id: Ib7216be0b84cca53a5681e15da2932ac16c1b6c1