To accomplish this, it was necessary to rearrange the tests so that
tests that ran servers in threads rather than using multiprocessing
into their own layer.  This is due to a bug currently in uvloop that
prevents using running uvloop servers in a process and in subprocesses
created with multiprocessing:

https://github.com/MagicStack/uvloop/issues/39

To run the tests with uvloop installed, it's necessary to use the
``-j`` option to run layers in separate processes.