[libcxx-dev] Broken libcxx-libcxxabi-libunwind bots due to Filesystem tests

[Duncan Exon Smith via libcxx-dev]

> It looks like the following bots assigned to you started failing recently:

I agree the bots should be fixed, but...

> I strongly suspect this is due to the fact that your builder is running as root. Libc++'s filesystem tests are known to fail when run as root, because some of them need to check for failure to access some files for which there's no permission. This doesn't seem to work when run as root.

It seems like these tests could have a line like:
```
REQUIRES: -user=root
```
and libcxx's lit target could export a `user=root` feature when tests were being run as root.  WDYT?

WDYT?