Unify workflow files

Author: hsbt

.github/workflows/ubuntu.yml renamed to .github/workflows/test.yml

@@ -4,9 +4,10 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
+        os: [ 'ubuntu-latest', 'macos-latest', 'windows-latest' ]
         ruby: [ 2.6, 2.5, 2.4, 2.3, 2.2, jruby, jruby-head, truffleruby, ruby-head ]
     steps:
     - uses: actions/checkout@v2