Make bridge test more robust

This branch tries to make BridgeTest more robust by testing each bridge multiple times and only considering a bridge to be unreachable if it fails every time.

Previously some unreliable obfs4 bridges were randomly passing or failing because our timeout (5 min) was just long enough for Tor to fail to bootstrap, wait, and retry. For a given bridge, sometimes the retry would complete before the timeout and sometimes not. I've reduced the timeout for obfs4 bridges to 2 min so we don't wait and retry (the test has its own retry mechanism already). Since meek is very slow I've increased its timeout to 6 min instead.

Even with 5 retries we're still seeing different bridges failing each time the test is run, so the test now tolerates up to 4 unreachable bridges. I've removed some consistently failing bridges and added a couple more from Tor Browser.

BridgeTest failures block the release pipeline so I'm assigning this to the current milestone and Sponsor 6.