[libcxx] [test] Avoid race conditions between tests regarding temp directories

Prior to e0d01294, all tests used a
random directory name, but now it is deterministic, based on the
test name. This change was done under the assumption that the filename
portion of the cwd is unique across tests that use the filesystem
test temporary directories.

When running tests locally, the cwd of the test is something like
"<build-dir>/test/<test path>/Output/copy_assign.pass.cpp.dir",
and the filename portion, "copy_assign.pass.cpp.dir", is used as
base for the temp directory names.

The change noted that there's a risk for race conditions if multiple
threads within one test try to create temp directories in parallel, but
that doesn't really happen in practice.

However, if running tests with a large number of parallel workers,
multiple tests with the same filename portion, e.g. "copy_assign.pass.cpp.dir",
can run in parallel, leading to race conditions across processes.

Therefore, add a hash of the full cwd to distinguish such cases
from each other.

Secondly, don't use two separate levels of temporary directories
(<base>/static_env.0). When cleaning up, only the individual
directory is removed, leaving the empty intermediate directory
behind littering the temp directory.

Differential Revision: https://reviews.llvm.org/D98703