[libc++] Use explicit module cache path in tests

For reference, this test creates about 1.5G in the cache
directory. By default this will go to ~/.cache/clang/
which can fill up quick. This changes the test to put the
cache path in lit temp directories. Size considerations
aside it makes sense for tests to be hermetic and not
touch global system state.

Differential revision: https://reviews.llvm.org/D127587