[libc++] Clean up tests for "optional" C11 features

First, add a TEST_HAS_QUICK_EXIT macro to mirror other C11 features like
TEST_HAS_ALIGNED_ALLOC, and update the tests for that.

Second, get rid of TEST_HAS_C11_FEATURES and _LIBCPP_HAS_C11_FEATURES,
which were only used to ensure that feature macros don't get out of
sync between <__config> and "test_macros.h". This is not necessary
anymore, since we have tests for each individual macro now.