perf(test): remove redundant integration test executions (#972)

## Summary

- Skip LLVM backend `test_exec_smir` tests in CI — keep Haskell only,
since it's the backend used for proving and bugs there have higher
impact
- Skip `test_prove_termination` in CI — the same 19 programs are already
executed via `test_exec_smir[*-haskell]`

This is done via a pytest `-k` filter in the CI workflow only — **no
test code is modified**, so all tests remain available for local
development.

**Deselected: 58 of 247 tests** (39 LLVM exec_smir + 19
prove_termination)

## Risk Analysis

- LLVM backend regression will be missed in the previous test, which
should be handled by future test framework refactoring. But if we don't
add new `exec_smir` test or add new `exec_smir` test with llvm to update
expected files, the result is the same as we run CI before.
- `test_prove_termination` just uses `prove-rs` to validate the
termination, which is the same as the comparison with the current
expected files. I believe that there is no risk to remove them in this
way.

## Expected CI time reduction

~2h37m → ~1h20m (based on [this
run](https://github.com/runtimeverification/mir-semantics/actions/runs/22658250856/job/65672639933?pr=957))

## Test plan

- [x] Integration tests pass with the `-k` filter applied
- [x] Verify CI time improvement

Resolves #971 (Phase 1)

---------

Co-authored-by: Everett Hildenbrandt <everett.hildenbrandt@gmail.com>
Co-authored-by: dkcumming <daniel.cumming@runtimeverification.com>

Author: 3 people

.github/workflows/test.yml

@@ -47,12 +47,37 @@ jobs:
         with:
           version: ${{ steps.uv_release.outputs.uv_version }}
       - name: 'Run unit tests'
-        run: make test-unit
+        run: make test-unit PARALLEL=12
 
   integration-tests:
     needs: code-quality-checks
-    name: 'Integration Tests'
+    name: 'Integration Tests ${{ matrix.name }}'
     runs-on: [self-hosted, linux, normal]
+    timeout-minutes: ${{ matrix.timeout }}
+    strategy:
+      fail-fast: true
+      matrix:
+        include:
+          - name: 'LLVM Concrete Tests'
+            test-args: '-k "llvm or test_run_smir_random"'
+            parallel: 12
+            timeout: 20
+          - name: 'Haskell Exec SMIR'
+            test-args: '-k "test_exec_smir and haskell"'
+            parallel: 6
+            timeout: 20
+          - name: 'Haskell Termination'
+            test-args: '-k test_prove_termination'
+            parallel: 6
+            timeout: 20
+          - name: 'Haskell Proofs'
+            test-args: '-k "test_prove and not test_prove_termination"'
+            parallel: 6
+            timeout: 60
+          - name: 'Remainder'
+            test-args: '-k "not llvm and not test_run_smir_random and not test_exec_smir and not test_prove_termination and not test_prove"'
+            parallel: 6
+            timeout: 20
     steps:
       - name: 'Check out code'
         uses: actions/checkout@v4
@@ -66,7 +91,9 @@ jobs:
       - name: 'Build stable-mir-json and kmir'
         run: docker exec --user github-user mir-semantics-ci-${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='${{ matrix.test-args }}' PARALLEL=${{ matrix.parallel }}
       - name: 'Tear down Docker'
         if: always()
         run: docker stop --time 0 mir-semantics-ci-${GITHUB_SHA}