MFC r289172,r290254:

r289172:

Refactor the test/ Makefiles after recent changes to bsd.test.mk (r289158) and
netbsd-tests.test.mk (r289151)

• Eliminate explicit OBJTOP/SRCTOP setting
• Convert all ad hoc NetBSD test integration over to netbsd-tests.test.mk
• Remove unnecessary TESTSDIR setting
• Use SRCTOP where possible for clarity

r290254:

Remove unused variable (SRCDIR)