Added a workflow to test on PHP 8 / Linux (#4790)

Currently we already test on PHP 8 on AppVeyor, but the failures there
may be windows-specific, so it makes sense to test on Linux as well

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: Run unit tests
+
+on: [push, pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.0'
+          tools: composer:v2
+
+      - uses: actions/checkout@v2
+
+      - name: Get Composer Cache Directories
+        id: composer-cache
+        run: |
+          echo "::set-output name=files_cache::$(composer config cache-files-dir)"
+          echo "::set-output name=vcs_cache::$(composer config cache-vcs-dir)"
+
+      - name: Cache composer cache
+        uses: actions/cache@v2
+        with:
+          path: |
+            ${{ steps.composer-cache.outputs.files_cache }}
+            ${{ steps.composer-cache.outputs.vcs_cache }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-composer-
+
+      - name: Run composer install
+        run: composer install -o
+        env:
+          COMPOSER_ROOT_VERSION: dev-master
+
+      - name: Run unit tests
+        run: php vendor/bin/paratest --runner=WrapperRunner