fix race in test

Author: bigbrett

tools/whnvmtool/Makefile

@@ -110,8 +110,10 @@ test-gen: $(OUT)
 	rm -f whNvmImage.bin whNvmImage.hex
 	./$(OUT) --test test/nvminit/test.nvminit --invert-erased-byte
 
-# Run the negative tests, where invalid configs and key files must fail the tool
-test-invalid: $(OUT)
+# Run the negative tests, where invalid configs and key files must fail the tool.
+# Order-only prereq on test-gen: every whnvmtool instance binds the same TCP
+# port, so the two tool-running targets must not overlap under make -j.
+test-invalid: $(OUT) | test-gen
 	./test/test_invalid_input.sh ./$(OUT)
 
 # Run the test suite, which requires first generating the test NVM image