A test pins stopping a mirror: only the repository's admin can, anyone else is told not found, and the target stays until then #6826

stopMirroring in src/actions/mirror_target.ts is the only way out of a mirror (#3408); no test imports it.

Done, in test/mirror-target.test.ts:

• [ ] The repository's admin stops it: the mirror target row is gone and the answer is ok(true); a second stop answers ok(false).
• [ ] A writer who is not admin, a stranger and a guest each get not_found, and the target row is still there.
• [ ] The non-admin assertion fails if requireRepoAdmin is skipped.

Files: test/mirror-target.test.ts only. Suggested: scout.