Reenable CI for macOS

Use Swift 5.2

Author: mattt

.github/workflows/ci.yml

@@ -3,23 +3,25 @@ name: CI
 on: [push]
 
 jobs:
-  # macos:
-  #   runs-on: macOS-latest
+  macos:
+    runs-on: macOS-latest
 
-  #   steps:
-  #     - name: Checkout
-  #       uses: actions/checkout@v1
-  #     - name: Build and Test
-  #       run: swift test
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v1
+      - name: Build and Test
+        run: swift test
+        env:
+          DEVELOPER_DIR: /Applications/Xcode_11.4.app/Contents/Developer
 
   linux:
     runs-on: ubuntu-latest
 
     container:
-      image: swiftlang/swift:nightly-5.2-xenial
+      image: swiftlang/swift:5.2
 
     steps:
       - name: Checkout
         uses: actions/checkout@v1
-      - name: Build and Run
-        run: swift run swift-doc Sources/SwiftDoc
+      - name: Build and Test
+        run: swift test --enable-test-discovery