test/ci: vendor clojure-test-suite as a submodule; run it in CI

The cross-dialect clojure-test-suite (jank-lang fork) is now a git submodule at
vendor/clojure-test-suite instead of a personal ~/src checkout. The harness reads
it from the vendored path only (skips cleanly if the submodule isn't initialized:
`git submodule update --init`).

CI already checks out submodules recursively (for vendor/sci), so the suite is now
fetched and — since `jpm test` recurses through test/ — the baseline (3981 pass /
66 clean) is enforced on every push/PR. Updated the checkout comment accordingly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/tests.yml

@@ -13,7 +13,9 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          # vendor/sci is needed by the SCI bootstrap/runtime integration tests.
+          # Submodules: vendor/sci (SCI bootstrap/runtime tests) and
+          # vendor/clojure-test-suite (the cross-dialect conformance battery,
+          # asserted against a baseline by clojure-test-suite-test.janet).
           submodules: recursive
 
       - name: Cache Janet build