Unit tests for installable plugins

[simonw]

I'd like more thorough unit test coverage of the plugins mechanism - in particular for installable plugins.

I think I can do this while still having the code live in the same repo, by creating a subdirectory in tests/example_plugin with its own setup.py and then running python setup.py install as part of the test runner.

I imagine I will need to bump the version number every time I change the plugin in case someone runs the test again in the same virtual environment.

If that doesn't work I can instead ship a datasette-plugins-tests two to PyPI and add that as a tests_require dependency.

Refs #14

[simonw]

The mechanism I described here - having a tests/example_plugin folder - is probably the right solution for #517

[simonw]

This is well handled now: https://github.com/simonw/datasette/tree/0.51.1/tests/plugins