Support running tests in parallel

Using the `pytest-xdist` plugin, this supports running the tests in parallel.  There are fancy distributed things `xdist` can do as well, but this just makes it easy to make the tests run faster locally on a multicore machine.

The `table` tests needed to be modified in order to not use global state which caused *random* problems.  The `timer` test was merged into a single test, since they must run in order and on the same core in order to succeed.