Make test CFLAGS separate from normal ones.

On Mac OS X Mavericks, we need to set -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk, but only for the tests that we run through SOAAP, not on the SOAAP build itself.

Make this distinction clearer and allow for future C++ tests by using TEST_C_FLAGS instead of CMAKE_C_FLAGS.