MFC r284408,r289151,r289158:

r284408:

Ensure TESTSDIR is defined before bsd.test.mk is .include'd

r289151:

Simplify netbsd-tests.test.mk

• projects/bmake and subsequent commits provide SRCTOP; there's no need to manually specify it now.
• Compute a sane default for OBJTOP based on .OBJDIR and RELDIR. Manually specifying this is probably no longer needed, but it persists just in case (supporting commits will need to be made to move it out of some of the meta .mk files).
• Compute a sane default for TESTSRC. Error out if the path cannot be found.

Sponsored by: EMC / Isilon Storage Division

r289158:

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
Sponsored by: EMC / Isilon Storage Division