Fix build in newer Swift versions

Author: yim-lee

docker/docker-compose.2004.56.yaml

@@ -11,3 +11,5 @@ services:
 
   test:
     image: swift-statsd-client:20.04-5.6
+    environment:
+      - FORCE_TEST_DISCOVERY=--enable-test-discovery

docker/docker-compose.2004.57.yaml

@@ -10,3 +10,5 @@ services:
 
   test:
     image: swift-statsd-client:20.04-5.7
+    environment:
+      - FORCE_TEST_DISCOVERY=--enable-test-discovery

docker/docker-compose.2004.main.yaml

@@ -10,3 +10,5 @@ services:
 
   test:
     image: swift-statsd-client:20.04-main
+    environment:
+      - FORCE_TEST_DISCOVERY=--enable-test-discovery

docker/docker-compose.yaml

@@ -28,4 +28,4 @@ services:
 
   test:
     <<: *common
-    command: /bin/bash -cl "swift test"
+    command: /bin/bash -cl "swift test $${FORCE_TEST_DISCOVERY-}"