[testsuite] Split Objective-C data formatter

The testcase for objective-c data formatters is very big as it checks a
bunch of stuff. This is annoying when using the lit test driver, because
it prevents us from running the different cases in parallel. As a
result, it's always one of the last few tests that complete. This patch
splits the test into multiple files that share a common base class. This
way lit can run the different tests in parallel.

Differential revision: https://reviews.llvm.org/D60300

llvm-svn: 357786