Revert "change test build configuration from Debug to Release"

This reverts commit a0ecc61501.

playground/ios/playground.xcodeproj/xcshareddata/xcschemes/playground.xcscheme

       </BuildActionEntries>
    </BuildAction>
    <TestAction
-      buildConfiguration = "Release"
+      buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       shouldUseLaunchSchemeArgsEnv = "NO">