[testsuite] xfailed two tests that depend on NSString conforming to NSCopying.

NSString is loaded from the DWARF, which doesn't have the concept of protocols.
When this is used with the NSMutableDictionary type from Objective-C modules,
this produces errors of the form

error: cannot initialize a parameter of type 'id<NSCopying> _Nonnull' with an rvalue of type 'NSString *'

We're aware of these problems and have an internal bug report filed
(<rdar://problem/32777981>)

llvm-svn: 305424