build: make check: use a PATH search instead for Kyua

which(1) accepts both relative/absolute paths as well as lone binary
names.  Set KYUA to kyua and use which(1) to confirm that it can find one;
if it cannot, just advise the user to set KYUA directly to the kyua binary
rather than assuming a relative location from LOCALBASE.

This allows `make check` to be operated with the version of kyua in base
without losing the flexibility of specifying another one.

ngie@ notes that the original intention was to avoid redundant $PATH lookups
and improve the determinism of the target. A future change will likely push
us back to this state, perhaps in the form of reverting this entirely and
just switching to using kyua in base. Accepting any in $PATH should be
considered a transitional move, at least until it's declared otherwise,
since kyua was only semi-recently added to base.

Reviewed-by:	brooks, emaste, lwhsu, ngie
Differential-Revision:	https://reviews.freebsd.org/D28045

share/mk/suite.test.mk

@@ -77,7 +77,7 @@ Kyuafile: Makefile
 	@mv ${.TARGET}.tmp ${.TARGET}
 .endif
 
-KYUA= ${LOCALBASE}/bin/kyua
+KYUA?=	kyua
 
 # Definition of the "make check" target and supporting variables.
 #
@@ -90,13 +90,13 @@ KYUA= ${LOCALBASE}/bin/kyua
 # report bogus results unless the new binaries are put in place.
 
 realcheck: .PHONY
-	@if [ ! -x ${KYUA} ]; then \
+	if ! which -s "${KYUA}"; then \
 		echo; \
 		echo "kyua binary not installed at expected location (${.TARGET})"; \
 		echo; \
 		echo "Please install via pkg install, or specify the path to the kyua"; \
-		echo "package via the \$${LOCALBASE} variable, e.g. "; \
-		echo "LOCALBASE=\"${LOCALBASE}\""; \
+		echo "binary via the \$${KYUA} variable, e.g., "; \
+		echo "KYUA=\"${LOCALBASE}/bin/kyua\""; \
 		false; \
 	fi
 	@env ${TESTS_ENV:Q} ${KYUA} test -k ${DESTDIR}${TESTSDIR}/Kyuafile