rtld-elf: Set proper TESTSDIR for tests/cheri and use TESTS_SUBDIRS

With the wrong TESTSDIR it picks up ${TESTSBASE}/libexec/rtld-elf,
racing with and potentially trampling over rtld-elf's own file. If
rtld-elf wins out then these tests are silently dropped, and if cheri
wins out then the file references the non-existent abi-mismatch/Kyafile
(relative to the rtld-elf directory itself).

We also need to use TESTS_SUBDIRS so rtld-elf's own Kyuafile recurses
into cheri, otherwise you have to run the tests separately.

Fixes:	41dda2a ("rtld-cheri-elf: Move tests to rtld-elf/tests/cheri")

libexec/rtld-elf/tests/Makefile

@@ -18,7 +18,7 @@ SRCS.$t=	$t.c common.c
 WARNS?=		3
 
 .if ${MACHINE_ABI:Mpurecap}
-SUBDIR+=	cheri
+TESTS_SUBDIRS+=	cheri
 .endif
 
 .include <bsd.test.mk>

libexec/rtld-elf/tests/cheri/Makefile

@@ -1,3 +1,5 @@
+TESTSDIR?=	${TESTSBASE}/libexec/rtld-elf/cheri
+
 TESTS_SUBDIRS+=	abi-mismatch
 
 .include <bsd.test.mk>