test-meta.sh: escape user/group expressions

Escape external user/group strings before passing them to 'grep -E',
otherwise calls like this will fail when the string includes regular
expression metacharacters:

Thanks to Pierre-Emmanuel Novac <piernov@gmail.com> for reporting the
problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>