run tests in parallel (#495)

Author: dnadoba

docker/docker-compose.yaml

@@ -28,7 +28,7 @@ services:
 
   test:
     <<: *common
-    command: /bin/bash -xcl "swift test -Xswiftc -warnings-as-errors $${SANITIZER_ARG-}"
+    command: /bin/bash -xcl "swift test --parallel -Xswiftc -warnings-as-errors $${SANITIZER_ARG-}"
 
   # util