ww driver: zero discovered tests is a loud failure

rundirtests returned rc=0 when it discovered no tests — a typo'd
path or empty corpus read as success. Fail loudly, matching the
cstage driver.

Makefile

@@ -266,6 +266,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_wwdumpgate_run \
 	  $(BIN)/test_datargate_run \
 	  $(BIN)/test_dynentry_run \
+	  $(BIN)/test_zerotest_run \
 	  $(BIN)/test_ampfncollide_run \
 	  $(BIN)/test_trycallcollide_run \
 	  $(BIN)/test_gunsigned_run \
@@ -860,6 +861,16 @@ $(BIN)/test_dynentry_run: test/wcc/989_dynentry_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+# 989_zerotest_run (#64, F15 c7): `ww test <dir>` over a directory with no
+# *_test.ww files fails loud (rc=1 + "no *_test.ww files in ...") instead of a
+# silent rc=0 false-green. Runs both driver twins (rule-10). See the header.
+$(BIN)/test_zerotest_run: test/wcc/989_zerotest_run.c \
+		$(BIN)/ww $(BIN)/ww_ww \
+		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 # 989_ampfncollide_run (#4, c2): `&fn` synthesis (unoptype TK_AMP +
 # assignableaddrfn) prefers the current module's fn when a same-leaf fn is
 # declared in a later module. Builds/rejects on BOTH driver twins (rule-10).