perf(ci): skip redundant integration tests via pytest -k filter

Add TEST_ARGS to the CI integration test step to skip:
- test_exec_smir[*-llvm]: keep Haskell backend only, since it's the
  backend used for proving and bugs there have higher impact
- test_prove_termination: the same 19 programs are already executed
  via test_exec_smir[*-haskell]

This deselects 58 of 247 tests (39 LLVM exec + 19 prove_termination)
without modifying any test code — tests remain available for local use.

Expected CI time reduction: ~2h37m → ~1h20m.

Resolves #971 (Phase 1)

Author: Stevengre

.github/workflows/test.yml

@@ -66,7 +66,9 @@ jobs:
       - name: 'Build stable-mir-json and kmir'
         run: docker exec --user github-user mir-semantics-ci-${GITHUB_SHA} make build
       - name: 'Run integration tests'
-        run: docker exec --user github-user mir-semantics-ci-${GITHUB_SHA} make test-integration
+        run: |
+          docker exec --user github-user mir-semantics-ci-${GITHUB_SHA} make test-integration \
+            TEST_ARGS='-k not (test_exec_smir and llvm) and not test_prove_termination'
       - name: 'Tear down Docker'
         if: always()
         run: docker stop --time 0 mir-semantics-ci-${GITHUB_SHA}