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

At some point output from a test run got accidentally checked into
test_root/data/, which made it hard to fully gitignore the other
test-run artifacts that land there. Delete the orphan files and
switch the directory to allow-list mode: ignore everything by
default and re-include the video/ fixtures we want to keep on
purpose.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: feanil

.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/