review the list of blacklisted sytests

[richvdh]

Some of the sytests on the blacklist look to be due to bugs that have since been fixed

[DMRobertson]

We now think we've got the most frequent flakes squashed in matrix-org/sytest#1142 and matrix-org/sytest#1138.

[DMRobertson]

I've had a quick look and I can't see any blacklisted tests that we can rerun.

I found a couple of blacklist entries suspicious: here and here.