ci/tests: add workflow status job

This job will wait until all other jobs in the workflow have finished,
and serves as the point of truth for 'required checks.'

This is a lot better than our current workflow, as it both accounts for
skipped jobs/matrixes (which are considered successful), and obviates
the need to change the branch configurations.

.github/workflows/tests.yaml

@@ -54,8 +54,8 @@ jobs:
   lockfile:
     name: Check poetry.lock
     runs-on: ubuntu-latest
-    needs: changes
     if: needs.changes.outputs.project == 'true'
+    needs: changes
     steps:
       - uses: actions/checkout@v4
 
@@ -66,8 +66,8 @@ jobs:
   fixtures-pypi:
     name: Check fixtures (PyPI)
     runs-on: ubuntu-latest
-    needs: changes
     if: needs.changes.outputs.fixtures-pypi == 'true'
+    needs: changes
     steps:
       - uses: actions/checkout@v4
 
@@ -184,3 +184,17 @@ jobs:
         working-directory: ./poetry-plugin-export
 
       - run: git -C poetry-plugin-export diff --exit-code --stat HEAD
+
+  status:
+    name: Status
+    runs-on: ubuntu-latest
+    if: always()
+    needs:
+      - lockfile
+      - fixtures-pypi
+      - mypy
+      - pytest
+      - pytest-export
+    steps:
+      - if: contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled')
+        run: exit 1