ci: make unit and execution tests able to fail a PR

Both jobs were continue-on-error: true at the job level, so a failing
pytest run reported success. They are the only two jobs in the workflow
tree that do this.

Verified green before removing the flag: the last 30 completed runs of
each workflow on master (90 matrix jobs each, ubuntu/windows/macos) have
zero failing steps.

Also sets fail-fast: false so one OS failing no longer cancels the other
two matrix legs' results.

.github/workflows/test-execution.yml

@@ -9,10 +9,10 @@ on:
 jobs:
   test:
     strategy:
+      fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
     runs-on: ${{ matrix.os }}
-    continue-on-error: true
     steps:
     - uses: actions/checkout@v4
     - name: Set up Python      

.github/workflows/test-unit.yml

@@ -9,10 +9,10 @@ on:
 jobs:
   test:
     strategy:
+      fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-2022, macos-latest]
     runs-on: ${{ matrix.os }}
-    continue-on-error: true
     steps:
     - uses: actions/checkout@v4
     - name: Set up Python