[TESTING] Expands on partly testing to include hedy.app and /tests fo…

…lder (hedyorg#3623)

**Description**
In this PR we further improve the testing flow to make sure GitHub performs some smart testing to decrease the time before a PR is validated. We add `hedy.app` and the `tests` folder to the changes that will trigger all tests and we also check if the `force_tests` label is active on the PR, if so: run all tests as well.

**Fixes**
This PR fixes hedyorg#3607

**How to test**
This should be fine as long as the tests run. Once the label is implemented, verify this as well.

.github/workflows/unittests.yml

@@ -41,12 +41,12 @@ jobs:
       id: check_file_changed
       run: |
         $diff = git diff --name-only ${{ github.event.pull_request.base.sha }} ${{ github.sha }}
-        $SourceDiff = $diff | Where-Object { $_ -match '^content/' -or $_ -match '^grammars/' -or $_ -match '^highlighting/' }
+        $SourceDiff = $diff | Where-Object { $_ -match '^content/' -or $_ -match '^grammars/' -or $_ -match '^highlighting/' -or $_ -match '^tests/' -or $_ -match '^app.py' -or $_ -match '^hedy.py'}
         $HasDiff = $SourceDiff.Length -gt 0
         Write-Host "::set-output name=docs_changed::$HasDiff"
     - name: Run all tests
       shell: pwsh
-      if: steps.check_file_changed.outputs.docs_changed == 'True'
+      if: steps.check_file_changed.outputs.docs_changed == 'True' -or ${{ github.event.label.name == 'force_tests' }}
       run: |
         build-tools/github/validate --all
     - name: Run non-code related tests