Update default.xml

Begin cloning swift-testing as this is now exploratory work.

Author: compnerd

default.xml

@@ -39,6 +39,7 @@
   <project remote="github" name="apple/swift-protobuf" path="swift-protobuf" groups="notdefault" />
   <project remote="github" name="apple/swift-syntax" path="swift-syntax" />
   <project remote="github" name="apple/swift-system" path="swift-system" revision="refs/tags/1.1.1" />
+  <project remote="github" name="apple/swift-testing" path="swift-testing" />
   <project remote="github" name="apple/swift-tools-support-core" path="swift-tools-support-core" />
   <project remote="github" name="apple/swift-xcode-playground-support" path="swift-xcode-playground-support" groups="notdefault" />