regex quoting in t/20-add.t

Hi Jeffrey,

On Mac OS-X one test using unescaped regexes for temporary paths sometimes fails, because sometimes paths contain "+" symbols and might look like:

/var/folders/Gr/GrrPQglwG7uGl2Ox6RCFD++++TU/-Tmp-/uZHXRZrOWA/authors/id/M/ME/ME/CHECKSUMS should exist

This pull requests puts \Q ... \E around the parts to make this test pass on OS-X. I do not see any negative side effects on other unix-flavours.

Best,
Wolfgang