[libcxx] Don't assume cwd name in std::filesystem tests

Summary:
In `std::filesystem::proximate` tests we assume that the current working directory's name
is `fs.op.proximate`. This is fine when we're running the tests locally.

However, if we're running those tests on a remote machine via SSH, the directory layout may be
different. For example, currently we copy each test executable individually into
a temporary directory on the target board using SCP, so the assumption about the working directory name
doesn't necessarily hold.

This patch is the only thing that is necessary for all libc++ tests to pass when run remotely.

Reviewers: ldionne, EricWF, mclow.lists

Reviewed By: ldionne, EricWF

Subscribers: christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D74348