Fix building/installing tests when TESTSBASE != /usr/tests

The work in r258233 hardcoded the assumption that tests was the last component
of the tests tree by pushing tests as an explicit prefix for the paths in
BSD.tests.dist and /usr was the prefix for all tests, per BSD.usr.dist and all
of the mtree calls used in Makefile.inc1. This assumption breaks if/when one
provides a custom TESTSBASE "prefix", e.g. TESTSBASE=/mytests .

One thing that r258233 did properly though was remove "/usr/tests" creation
from BSD.usr.dist -- that should have not been there in the first place. That
was an "oops" on my part for the work that was originally committed in r241823

Phabric: D1301
Reviewed by: imp
Sponsored by: EMC / Isilon Storage Division