[libc++] Refer to the Filesystem static test env as relative paths

[libc++] Refer to the Filesystem static test env as relative paths

Instead of hardcoding absolute paths on the build-host in the executables,
use relative paths from the current working directory. Also, use
FILE_DEPENDENCIES to mark the static test env as being required by
the relevant tests.

Given a SSH executor that copies the files to the remote host properly,
the tests can be run on that remote host.