Fix racy callback test
Fix racy callback test

ThreadedCallbackValue1Ordered test was designed racy because
RemoveCallback() can be called before the callback to-be-removed
is added.

Change the test so that the to-be-removed callback is added before
removal.

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk