test: prove restic check actually fails on a corrupted repository

compat.sh only grepped the check output for "Check Successful". check.sh
prints that string whenever restic exits 0, so a `restic check` that had
been turned into a no-op kept the suite green - the integrity check, half
the purpose of this image, was untested.

Adds a section that backs up, overwrites the first pack file under
/mnt/restic/data with garbage and then requires the failure path:
"Check Failed with Status", no "Check Successful", and
/var/log/check-error-last.log written. It uses its own throwaway container
so the later tests do not inherit the broken repository.

Verification on Git-Bash:
  bash test/compat.sh                    -> 36 passed, 0 failed
  sabotage (check.sh: restic check -> true), rebuilt image
  IMAGE=rbd-sab-check bash test/compat.sh -> 33 passed, 3 failed, exit 1
    - restic check detects a corrupted repository
    - a corrupted repository is not reported as successful
    - failing check writes check-error-last.log
check.sh was restored with git checkout and the sabotage image removed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Agent-Id: agent-4966d0e6-7f4d-4b37-be3d-0648ed56cb8a

test/compat.sh

@@ -92,6 +92,33 @@ assert_contains "$hook_log" "POST-HOOK-RAN with rc=0" \
 
 stop_container
 
+echo
+echo "=== compat: a corrupted repository is reported as failed ==="
+# The success path above is not enough: check.sh prints "Check Successful"
+# whenever restic exits 0, so a `restic check` that does nothing at all would
+# keep this suite green. Damage the repository and require the failure path.
+# Runs in its own throwaway container, the repository stays broken afterwards.
+
+start_container corrupt
+wait_for_start || { fail "container starts for the corruption test"; summary "compat"; exit 1; }
+write_file /data/hello.txt "backup me"
+in_container /bin/backup >/dev/null 2>&1
+assert_eq "1" "$(snapshot_count)" \
+    "the repository to be corrupted has a snapshot"
+
+in_container sh -c 'f=$(find /mnt/restic/data -type f | head -n1); chmod u+w "$f"; printf garbage > "$f"'
+broken_check_log="$(in_container /bin/check 2>&1)"
+assert_contains "$broken_check_log" "Check Failed with Status" \
+    "restic check detects a corrupted repository"
+assert_not_contains "$broken_check_log" "Check Successful" \
+    "a corrupted repository is not reported as successful"
+if in_container test -f /var/log/check-error-last.log; then
+    pass "failing check writes check-error-last.log"
+else
+    fail "failing check writes check-error-last.log" "file is missing"
+fi
+stop_container
+
 echo
 echo "=== compat: RESTIC_FORGET_ARGS ==="