Add GitHub Actions job to test SwiftPM

Author: manicmaniac

.github/workflows/test.yml

@@ -43,6 +43,14 @@ jobs:
     - uses: actions/checkout@v2
     - name: Run install tests
       run: make -C InstallTests carthage
+  swiftpm-test:
+    runs-on: macOS-10.15
+    env:
+      DEVELOPER_DIR: "/Applications/Xcode_12.app/Contents/Developer"
+    steps:
+      - uses: actions/checkout@v2
+      - name: Run unit tests
+        run: swift test
   lint-podspec:
     runs-on: macOS-10.15
     env: