Support pytest-xdist

[jitseniesen]

Currently, tests can not be run in parallel using the pytest-xdist plugin. There are two problems:

• The plugin does not report tests as they are collected using the standard pytest hooks.
• When the tests are run in parallel, the three phases of the tests (setup, run, teardown) are interspersed and this confusing spyder-unittest.

We should make spyder-unittest more robust so that pytest-xdist can be used.