test, win: fix up symlink tests

On Windows, creating a symlink requires admin privileges.
There were two tests which created symlinks which were failing when run
as non-admin.

test-fs-symlink.js already had a check for privileges on Windows
but it had a couple issues:
1. It assumed that whoami was the one that came with windows.
   However, whoami also ships with Win32 Unix utility ports
   like the distribution with git, which can cause this to get check
   tripped up.
2. On failure, the check would just return from the callback instead of
   exiting
3. whoami was executed asynchronously so the test would run regardless
   of privilege state.

test-fs-options-immutable had no check.

As part of this change, I refactored the privilege checking to
a function in common, and changed both above tests to use the
refactored function.

Also documented this function in test\README.md

PR-URL: https://github.com/nodejs/node/pull/10477


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>