test: Update test README and lint script

doc/fuzzing.md

@@ -16,6 +16,9 @@ $ FUZZ=process_message src/test/fuzz/fuzz
 # abort fuzzing using ctrl-c
 ```
 
+There is also a runner script to execute all fuzz targets. Refer to
+`./test/fuzz/test_runner.py --help` for more details.
+
 ## Fuzzing harnesses and output
 
 [`process_message`](https://github.com/bitcoin/bitcoin/blob/master/src/test/fuzz/process_message.cpp) is a fuzzing harness for the [`ProcessMessage(...)` function (`net_processing`)](https://github.com/bitcoin/bitcoin/blob/master/src/net_processing.cpp). The available fuzzing harnesses are found in [`src/test/fuzz/`](https://github.com/bitcoin/bitcoin/tree/master/src/test/fuzz).