atf: refer to common.mk using SRCTOP

This helps make the dependent Makefile (and the values contained within)
easier to catch when upgrading atf in the future.

MFC after:	1 week

lib/atf/libatf-c++/tests/detail/Makefile

@@ -29,5 +29,5 @@ BINDIR.${p}=	${TESTSDIR}
 .endfor
 version_helper.o: atf-version
 
-.include "../../../common.mk"
+.include "${SRCTOP}/lib/atf/common.mk"
 .include <bsd.test.mk>

lib/atf/libatf-c/tests/detail/Makefile

@@ -31,5 +31,5 @@ BINDIR.${p}=	${TESTSDIR}
 .endfor
 version_helper.o: atf-version
 
-.include "../../../common.mk"
+.include "${SRCTOP}/lib/atf/common.mk"
 .include <bsd.test.mk>

libexec/atf/atf-sh/Makefile

@@ -75,5 +75,5 @@ SUBR=		libatf-sh.subr
 HAS_TESTS=
 SUBDIR.${MK_TESTS}+= tests
 
-.include "../../../lib/atf/common.mk"
+.include "${SRCTOP}/lib/atf/common.mk"
 .include <bsd.prog.mk>