acceptance: Improve the windows user check in facter/acceptance/test/facts/windows.rb

In FACT-1442 the regular expression used to validate the user identity from facter on windows was modified to be more broad. This was to expedite the CI pipelines passing due to Windows 10 being added. However the regex may be too broad and there value is never actually verified.

This ticket should;

• Review the regex and find one that's more specific to stop false positives (as per the comments in https://github.com/puppetlabs/facter/pull/1358#discussion_r68339551)

• Investigate whether it is possible to verify that the value returned from facter is actually true