Add supporting changes for Add limited sandbox capability to "make check"

Non-tests/... changes:
- Add HAS_TESTS= to Makefiles with libraries and programs to enable iteration
  and propagate the appropriate environment down to *.test.mk.

tests/... changes:
- Add appropriate support Makefile.inc's to set HAS_TESTS in a minimal manner,
  since tests/... is a special subdirectory tree compared to the others.

MFC after:	2 months
MFC with:	r322511
Reviewed by:	arch (silence), testing (silence)
Differential Revision:	D12014

libexec/atf/atf-check/Makefile

@@ -40,6 +40,7 @@ CFLAGS+=	-DATF_SHELL='"/bin/sh"'
 
 LIBADD=		atf_cxx
 
+HAS_TESTS=
 SUBDIR.${MK_TESTS}+= tests
 
 .include <bsd.prog.mk>

libexec/atf/atf-sh/Makefile

@@ -72,6 +72,7 @@ FILESGROUPS=	SUBR
 SUBRDIR=	${SHAREDIR}/atf
 SUBR=		libatf-sh.subr
 
+HAS_TESTS=
 SUBDIR.${MK_TESTS}+= tests
 
 .include "../../../lib/atf/common.mk"

libexec/rtld-elf/Makefile

@@ -85,6 +85,7 @@ beforeinstall:
 
 .PATH: ${.CURDIR}/${RTLD_ARCH}
 
+HAS_TESTS=
 SUBDIR.${MK_TESTS}+= tests
 
 .include <bsd.prog.mk>