test: add container test suites and run them in CI

The repository had no tests: CI only built the image, and the old
container-structure-test setup was parked as config-broken.yml /
docker-compose.test.broken.yml. This adds suites that actually run the image.

- test/compat.sh: backup, restore, forget, check, hooks, failure handling and
  timezone behaviour against the default configuration. These describe what
  users depend on today, so a change that turns them red is a breaking change.
- test/features.sh: optional features, each guarded by a detection check
  against the built image. Absent features are reported as SKIP, so open pull
  requests do not turn master red, and their feature tests start running by
  themselves once the pull request is rebased. Covers #109 (RESTIC_STREAM),
  #99 (docker cli, bash) and #84 (msmtp).
- test/required-features.txt: once a feature is merged its gate is listed here
  and a missing feature becomes a failure instead of a skip.
- test/structure.yml: revived image layout checks, falls back to the official
  container-structure-test image when the tool is not installed.
- test/run.sh --sabotage: breaks the backup call on a throwaway copy of the
  tree and expects the suite to fail, so the tests cannot be watered down into
  always passing. CI runs this as its own step.
- .github/workflows/test.yml: shellcheck plus all suites on every pull request.

Also adds .gitattributes pinning shell scripts, the Dockerfile and YAML to LF.
With a Windows checkout (core.autocrlf=true) the scripts were checked out with
CRLF, and the resulting image failed to start with
"exec /entry.sh: no such file or directory".

No runtime file is touched, the image itself is unchanged.

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

Agent-Id: agent-7034f75b-1ddc-4aa1-a4ea-3a54b340e8d9

test/required-features.txt

@@ -0,0 +1,18 @@
+# Optional features that MUST be present in the built image.
+#
+# One gate name per line. Known gates and the pull request that introduces
+# them (see test/features.sh):
+#
+#   stream       stream backups via RESTIC_STREAM            PR #109
+#   docker-cli   docker cli inside the container             PR #99
+#   bash         bash for pre/post hooks                     PR #99
+#   msmtp        mail relay via msmtp                        PR #84
+#
+# While a gate is NOT listed here its tests are reported as SKIP when the
+# feature is absent, so an open pull request does not turn master red.
+#
+# When you merge a feature, add its gate name below. From then on the feature
+# tests are mandatory and a feature that silently disappears from the image
+# fails the build.
+#
+# Empty on purpose: master currently ships none of the optional features.