Disable parallel tests in BuildTool command.

Author: maratal

Sources/BuildTool/XcodeRunner.swift

@@ -15,6 +15,7 @@ enum XcodeRunner {
 
         arguments.append(contentsOf: ["-scheme", scheme])
         arguments.append(contentsOf: ["-destination", destination.xcodebuildArgument])
+        arguments.append(contentsOf: ["-parallel-testing-enabled", "NO"])
 
         /*
          Note: I was previously passing SWIFT_TREAT_WARNINGS_AS_ERRORS=YES here, but am no longer able to do so, for the following reasons: