Update GitHub Actions for Laravel 8

Author: laravel-shift

.github/workflows/run-tests.yml

@@ -10,9 +10,11 @@ jobs:
             matrix:
                 os: [ubuntu-latest, windows-latest]
                 php: [7.4]
-                laravel: [5.8.*, 6.*, 7.*]
+                laravel: [8.*, 5.8.*, 6.*, 7.*]
                 dependency-version: [prefer-lowest, prefer-stable]
                 include:
+                    -   laravel: 8.*
+                        testbench: 6.*
                     -   laravel: 7.*
                         testbench: 5.*
                     -   laravel: 6.*