Fix mq.datastore test

[Maks-s]

Hello, awesome software :+1: !

I couldn't get the test to work no matter what I tried. Commit 687d1f0 helped, it made the test pass 9 times out of 10

This PR makes it work 10/10, but github loves to blow up when someone changes an indent so here's a rundown of what this PR does

• Replace sleep with FakeTime
• Use try/finally to remove tmpFile and close the db even when a test fails
• Wait for the queue to be empty before running the 'subscribe' assert (see line 54)

If I may also suggest, BDD testing could be nice to better track and fix failing tests

[zefhemel]

Thanks for this!

Regarding BDD: I'm open to it. For me the current approach has been ok, but open to contributions!