build-sys: fix make check when polkit not available

acl-helper needs polkit (and unix atm) to build.
Signed-off-by: Marc-André Lureau <marcandre.lureau@gmail.com>

tests/Makefile.am

 NULL =
 
+noinst_PROGRAMS =
 TESTS = coroutine				\
 	util					\
 	session					\
-	usb-acl-helper				\
 	$(NULL)
 
 if WITH_PHODAV
 TESTS += pipe
 endif
 
-noinst_PROGRAMS = $(TESTS) \
-		  mock-acl-helper \
-		  $(NULL)
+if WITH_POLKIT
+TESTS += usb-acl-helper
+noinst_PROGRAMS += mock-acl-helper
+endif
+
+noinst_PROGRAMS += $(TESTS)
 
 AM_CPPFLAGS =					\
 	$(COMMON_CFLAGS)			\