Add test workflow

.github/workflows/dockerimage.yml

@@ -1,5 +1,9 @@
 name: Docker Image CI
-on: [push]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 jobs:
   build:
     runs-on: ubuntu-latest

.github/workflows/test.yml

@@ -0,0 +1,20 @@
+name: Test
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ./
+        id: autobump
+        with:
+          file: testdata/testfile
+          version_name: REVIEWDOG_VERSION
+          repo: reviewdog/reviewdog
+
+      - name: Check diff
+        run: git diff