Added a "Start Database" step to github action flow

https://github.blog/changelog/2020-02-21-github-actions-breaking-change-ubuntu-virtual-environments-will-no-longer-start-the-mysql-service-automatically/

.github/workflows/phpunit.yml

@@ -31,6 +31,10 @@ jobs:
         path: ${{ steps.composer-cache.outputs.dir }}
         key: ${{ runner.os }}-composer-${{ matrix.php }}
 
+    - name: Start Database
+      run: |
+        sudo /etc/init.d/mysql start
+
     - name: Setup Database
       run: |
         mysql -uroot -proot -e 'CREATE DATABASE IF NOT EXISTS `bookstack-test`;'