Expose _TESTS_USE_OBJDIR as MK_MAKE_CHECK_USE_SANDBOX and anchor the default

value on MK_TESTS.

Use bsd.opts.mk in suite.test.mk to toggle the behavior.

share/mk/bsd.lib.mk

@@ -443,7 +443,7 @@ OBJS_DEPEND_GUESS.${_S:R}.pico+=	${_S}
 .endif
 
 .if defined(HAS_TESTS)
-MAKE+=			-D_TESTS_USE_OBJDIR
+MAKE+=			MK_MAKE_CHECK_USE_SANDBOX=yes
 SUBDIR_TARGETS+=	check
 TESTS_LD_LIBRARY_PATH+=	${.OBJDIR}
 .endif

share/mk/bsd.opts.mk

@@ -55,6 +55,7 @@ __DEFAULT_YES_OPTIONS = \
     INCLUDES \
     INSTALLLIB \
     KERBEROS \
+    MAKE_CHECK_USE_SANDBOX \
     MAN \
     MANCOMPRESS \
     NIS \
@@ -75,6 +76,7 @@ __DEFAULT_NO_OPTIONS = \
 
 # meta mode related
 __DEFAULT_DEPENDENT_OPTIONS = \
+    MAKE_CHECK_USE_SANDBOX/TESTS
     STAGING_MAN/STAGING \
     STAGING_PROG/STAGING \
     STALE_STAGED/STAGING \

share/mk/bsd.prog.mk

@@ -306,7 +306,7 @@ lint: ${SRCS:M*.c}
 .endif
 
 .if defined(HAS_TESTS)
-MAKE+=			-D_TESTS_USE_OBJDIR
+MAKE+=			MK_MAKE_CHECK_USE_SANDBOX=yes
 SUBDIR_TARGETS+=	check
 TESTS_LD_LIBRARY_PATH+=	${.OBJDIR}
 TESTS_PATH+=		${.OBJDIR}

share/mk/suite.test.mk

@@ -8,6 +8,8 @@
 .error suite.test.mk cannot be included directly.
 .endif
 
+.include <bsd.opts.mk>
+
 # Name of the test suite these tests belong to.  Should rarely be changed for
 # Makefiles built into the FreeBSD src tree.
 TESTSUITE?= FreeBSD
@@ -101,7 +103,7 @@ realcheck: .PHONY
 	fi
 	@env ${TESTS_ENV:Q} ${KYUA} test -k ${CHECKDIR}/Kyuafile
 
-.if defined(_TESTS_USE_OBJDIR) && make(check)
+.if ${MK_MAKE_CHECK_USE_SANDBOX} && make(check)
 DESTDIR:=	${.OBJDIR}/checkdir
 CLEANDIRS+=	${DESTDIR}