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.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division