MFC r305018,r305019,r305020:

r305018:

Use SRCTOP instead of a homegrown definition for it (SRCDIR)

r305019:

Remove unnecessary variable (SRCDIR) replaced by SRCTOP in Makefile.common

r305020:

Remove redundant declarations and simplify ../ in pathing

• TESTSBASE and LOCALBASE are already defined in bsd.tests.mk
• TESTSDIR is automatically divined as ${TESTSBASE}${RELDIR:H} after r289158.
• Replace SRCDIR with SRCTOP