Default TESTSDIR to /usr/tests/${RELDIR:H}

When run from bin/ls/tests, for example, the value of TESTSDIR would be
${TESTSBASE}/${RELDIR:H} -> /usr/tests/bin/ls/tests/.. ->
/usr/tests/bin/ls

Document the new behavior in bsd.README.

While here, also document TESTSBASE

Relnotes: yes
Differential Revision: D1022
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division