ci: add action for unittesting and creating cover badge

.github/workflows/actions.yml

@@ -0,0 +1,23 @@
+name: Unittest
+
+on: [push]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+  steps:
+    - uses: actions/checkout@v3
+    - uses: actions/setup-go@v3
+
+    - name: Run unittests
+      run: go test ./... -coverprofile=./cover.out -covermode=atomic -coverpkg=./...
+
+    - name: Generate cover badge
+      uses: vladopajic/go-test-coverage@v2
+      with:
+        profile: cover.out
+        locale-prefix: github.com/gkits/sqlnull
+        threshold-total: 95
+        git-token: ${{ github.ref_name == 'main' && secrets.GITHUB_TOKEN || '' }}
+        git-branch: badges