Add GitHub workflow 'coverage'

Creates a code coverage badge in .github/coverage.svg using PHPUnit on every push.

Author: PhrozenByte

.github/workflows/coverage.yml

@@ -0,0 +1,34 @@
+name: Coverage
+
+on:
+    push:
+        branches: [ master ]
+
+jobs:
+    run:
+        runs-on: ubuntu-latest
+
+        steps:
+            -   name: Checkout
+                uses: actions/checkout@v2
+
+            -   name: Setup PHP
+                uses: shivammathur/setup-php@v2
+                with:
+                    php-version: '7.2'
+                    coverage: xdebug
+
+            -   name: Install dependencies
+                run: composer update --prefer-dist --no-progress
+
+            -   name: Run PHPUnit test suite
+                run: ./vendor/bin/phpunit --configuration ./tests/phpunit.xml
+
+            -   name: Update code coverage badge
+                uses: timkrase/phpunit-coverage-badge@v1.0.0
+                with:
+                    report: ./tests/clover.xml
+                    report_type: clover
+                    coverage_badge_path: ./.github/coverage.svg
+                    push_badge: true
+                    repo_token: ${{ secrets.GITHUB_TOKEN }}