test: add gloss-drill test suite (red phase)

Four test files plus a small testutil for toggling the `org-drill'
feature flag. All 10 tests fail at this commit because the implementation
is still a stub.

The suite covers Normal (untagged entries get the tag and property),
Boundary (empty file, idempotency, untag never-tagged), and Error
(org-drill not installed). The error path also asserts the file is left
untouched. Untag-all is tested under both feature states because the
user might want to remove tags after uninstalling org-drill.

0 files changed, 0 insertions, 0 deletions