mtree: TESTSBASE directory always starts with a /

Remove the extra forward slash ("/"), otherwise the mtree specification
file will have the double slash and will not be parsed by makefs when
attempting to build NanoBSD with NO_ROOT privileges.

Fixes: 07670b30fa43 ("Create /usr/tests *.debug file directory hierarchy")
Reviewed by: emaste
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D47722