Bug 1370279 - Improve pytest test collection performance

Before this change running pytest with no arguments took 51 seconds
to just collect the list of tests to run (`pytest --collect-only`),
since the entire source directory was scanned (including the 18,000
files in `node_modules`).

Now it takes only 3.7 seconds.

setup.cfg

@@ -28,7 +28,8 @@ line_length = 140
 known_first_party = tests
 
 [tool:pytest]
-norecursedirs = jenkins
+testpaths = tests
+norecursedirs = __pycache__ jenkins ui
 DJANGO_SETTINGS_MODULE=tests.settings
 markers =
     slow: mark a test as slow.