Update sys.platform switched behavior in tests to use self.getPlatform (remote target platform)

Uses target platform when determining which platform specific behavior to use
or expect in tests. TestHelp.py was unchanged because this is asserting
behavior of the local lldb binary.

Test Plan:
Run tests on different remote os. Several previously failing tests now pass:
TestArrayTypes.py
TestInferiorChanged.py
TestInferiorCrashing.py
TestIvarProtocols.py
TestProcessIO.py
TestPublicAPIHeaders.py
TestRecursiveInferior.py

Differential Revision: http://reviews.llvm.org/D8747

llvm-svn: 233805