chore: clean up test_root/data/ and ignore generated artifacts

.gitignore

@@ -89,6 +89,14 @@ pytest_task*.txt
 common/test/data/test_unicode/static/
 test_root/courses/
 test_root/data/test_bare.git/
+# `test_root/data/` is `settings.GITHUB_REPO_ROOT` in the test settings;
+# course-import code (`cms/djangoapps/contentstore/.../tasks.py::import_olx`)
+# writes per-course subdirectories here named `urlsafe_b64encode(repr(course_key))`.
+# Ignore everything in that directory by default and explicitly re-include the
+# fixtures we actually want tracked. New fixture additions need an extra `!` line
+# below.
+test_root/data/*
+!test_root/data/video
 test_root/export_course_repos/
 test_root/paver_logs/
 test_root/uploads/