tools/test/Makefile: fix always-on generation of tools/test/gen_test.c

We need to tell it that the tmp file is an intermediate, so doesn't need
remaking if it doesn't exist.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/test/Makefile

@@ -26,6 +26,7 @@ $(TOOL_TEST_PROGRAMS): $(TOOL_TEST_COMMON_OBJS) $(TOOL_GEN_SRC:.c=.o) tools/test
 tools/test/gen_test.h: $(TOOLS_WIRE_DEPS)
 	$(BOLT_GEN) --page header $@ test_type < tools/test/test_cases > $@
 
+.INTERMEDIATE: tools/test/gen_test.c.tmp.c
 # Parallel make sometimes tries to use file before update-mocks, so split.
 tools/test/gen_test.c.tmp.c: $(TOOLS_WIRE_DEPS)
 	$(BOLT_GEN) --page impl tools/test/gen_test.h test_type < tools/test/test_cases > $@