Add unit test for "static" mechanism in plugins

[simonw]

Split out from #272 - this is actually quite tricky. Here's the relevant code:

datasette/datasette/utils.py

Lines 602 to 614 in 35429f9

	for plugin in pm.get_plugins():
	static_path = None
	templates_path = None
	try:
	if pkg_resources.resource_isdir(plugin.__name__, "static"):
	static_path = pkg_resources.resource_filename(plugin.__name__, "static")
	if pkg_resources.resource_isdir(plugin.__name__, "templates"):
	templates_path = pkg_resources.resource_filename(
	plugin.__name__, "templates"
	)
	except (KeyError, ImportError):
	# Caused by --plugins_dir= plugins - KeyError/ImportError thrown in Py3.5
	pass

[simonw]

See also #226 - "Unit tests for installable plugins"