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.
