Fix "make WINDOWS_BUILD=1 clean" on non-Windows hosts

The clean rule was not using the correct names for the compiled
executable files.
diff --git a/tests/Makefile b/tests/Makefile
index 4d2edd4..b6e49bf 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -105,7 +105,7 @@
 
 clean:
 ifndef WINDOWS
-	rm -rf $(APPS) *.c *.datax TESTS
+	rm -rf $(BINARIES) *.c *.datax TESTS
 else
 	del /Q /F *.c *.exe *.datax
 ifneq ($(wildcard TESTS/.*),)