This is because the assertion utility functions used in testCFStrings() expects to get the JSGlobalContextRef from the global context variable. However, testCFStrings() creates its own JSGlobalContextRef but does not set the global context variable to it.
Created attachment 311363 [details] proposed patch.
Thanks for the review. Landed in r217498: <http://trac.webkit.org/r217498>.
<rdar://problem/32479707>