Bug 1433390 - Use register_assert_rewrite to enable introspection in assertion helper. r=jgraham

Pytest by default only loads test modules, and enables assert
introspection for them. For any helper method which also uses
asserts, only a plain assertion is printed.

To enable introspection also for the assertion helper, the
module has to be registered for assertion rewrite.

MozReview-Commit-ID: 97JzuykXxWE

--HG--
extra : rebase_source : b83516bbb943d36b76a344b36b5ac9d508eba3d0

testing/web-platform/tests/webdriver/tests/__init__.py

@@ -0,0 +1,4 @@
+import pytest
+
+# Enable pytest assert introspection for assertion helper
+pytest.register_assert_rewrite('tests.support.asserts')