For now we set the FileSource for the Worker context
but in the future after fixing #1664 we can assert()
that FileSource are set only for the Map thread context.
mbgl::Thread will keep a ThreadContext for each running instance
in a thread_local so we don't need to lookup a man in the Environment
every time we need some info about the current thread.
This patch is moving the ::currentlyOn check used on Debug build from
the Environment class to the ThreadContext.
When testing failed vector tiles requests, we check for error messages
of a very specific tiles, but mbgl will requests 4 tiles.
We might get the error callback before the tile we are checking for
reports error, so the test fails.
Now we use a regex to match the log message we are looking for.
Test whenever destroying a Map object with requests pending
works and is done quickly.
The test injects artificial delays in selected requests and
tries to destroy the Map object afterwards.
Currently glyph requests are skipped because there is a bug
being worked on in a different issue.
