[libcxx] [test] Place output from newconfig tests under a 'test' subdir

Previously, all the output from the tests were placed directly in
the build directory. The tests produce a couple directories named
`__config_{exec,cache,src}__` which are easy to distinguish, but
the output from the individual tests are placed in subdirectories
named `std` or `libcxx`. Especially the build output from libcxx
tests ends up conflated in the same directories as are used for
building libcxx with cmake.

Differential Revision: https://reviews.llvm.org/D120479