Bug 1389517 - Make Python warnings fatal during pytest runs

This makes most warning types (including the normally hidden
`DeprecatonWarning`s) fatal during pytest runs, to help prevent
regressions now that most of them have been fixed.

See:
https://docs.pytest.org/en/latest/warnings.html
https://docs.python.org/2.7/library/warnings.html#warning-categories

setup.cfg

@@ -31,6 +31,17 @@ norecursedirs = __pycache__ jenkins ui
 DJANGO_SETTINGS_MODULE=tests.settings
 # Disable unused auto-loaded plugins.
 addopts = --driver Firefox -p no:mozlog -p no:metadata -p no:html
+# Make most warnings fatal (including the hidden by default DeprecationWarning):
+# https://docs.pytest.org/en/latest/warnings.html
+# https://docs.python.org/2.7/library/warnings.html#warning-categories
+# Note that some warnings are currently missed by pytest:
+# https://github.com/pytest-dev/pytest/issues/3251
+filterwarnings =
+    error
+    # TODO: Fix usages of `selenium` and remove this override.
+    default:use driver instead:DeprecationWarning:pypom.view
+    ignore::ImportWarning
+    ignore::PendingDeprecationWarning
 markers =
     slow: mark a test as slow.
 xfail_strict = true