test: only rerun the failed tests in CI

This patch simplifies attempting to execute the tests again on
failures in CI. It also limits the scope of the tests being rerun to
include only those that failed on the first try. Additionally, it
enables the maximum verbosity so that the retry is more useful for
troubleshooting.

Author: webknjaz

.github/workflows/testsuite.yml

@@ -74,23 +74,14 @@ jobs:
           # python -c "import urllib.request as r; exec(r.urlopen('https://bit.ly/pydoctor').read())"
 
       - name: "Run tox for ${{ matrix.python-version }}"
-        continue-on-error: true
-        id: tox1
         run: |
           python -m tox -- -rfsEX
 
       - name: "Retry tox for ${{ matrix.python-version }}"
-        id: tox2
-        if: steps.tox1.outcome == 'failure'
+        if: failure()
         run: |
-          python -m tox -- -rfsEX
-
-      - name: "Set status"
-        if: always()
-        run: |
-          if ${{ steps.tox1.outcome != 'success' && steps.tox2.outcome != 'success' }}; then
-             exit 1
-          fi
+          # `exit 1` makes sure that the job remains red with flaky runs
+          python -m tox -- -rfsEX --lf -vvvvv && exit 1
 
   # This job aggregates test results. It's the required check for branch protection.
   # https://github.com/marketplace/actions/alls-green#why