tests/Makefile.am: use LIBTOOL variable instead of hardcoded name

Fixes "can't find libtool" error when running valgrind checks.
Signed-off-by: NMaciej Wolny <maciej.wolny@codethink.co.uk>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

tests/Makefile.am

@@ -487,7 +487,7 @@ VALGRIND = valgrind --quiet --leak-check=full --trace-children=yes \
 	--trace-children-skip="*/tools/virsh","*/tests/commandhelper" \
 	--suppressions=$(srcdir)/.valgrind.supp
 valgrind:
-	$(MAKE) check VG="libtool --mode=execute $(VALGRIND)"
+	$(MAKE) check VG="$(LIBTOOL) --mode=execute $(VALGRIND)"
 
 sockettest_SOURCES = \
 	sockettest.c \