Detect when test suite runs with privileged access

Summary:
When the test suite runs as root under Linux or FreeBSD,
or with elevated status ('runs as administrator') on Windows,
the process can see normally non-readable files.

As such, we skip the relevant tests.

Test Plan:

• Windows: runs in a regular console and a privileged console
• Debian: runs as root, and with sudo -u nobody phpunit
• FreeBSD: runs trim(shell_exec('id -u')) with psysh

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2278