[analyzer] Teach ObjCDeallocChecker about XCTestCase

Like with SenTestCase, subclasses of XCTestCase follow a "tear down" idiom to
release instance variables and so typically do not release ivars in -dealloc.
This commit applies the existing special casing for SenTestCase to XCTestCase
as well.

rdar://problem/25884696

llvm-svn: 273441