test/fuzz: fix rpc, secret_connection and pex tests (#6190)

* test/fuzz: fix rpc, secret_connection and pex tests

- ignore empty data in rpc
- provide correct IP in pex
- spawn a goroutine for Write and do multiple Read(s)

* test/fuzz: fix init in pex test

* test/fuzz: assign NewServeMux to global var

* test/fuzz: only try to Unmarshal if blob is not empty

* run fuzz tests for PRs which modify fuzz tests themselves

* test/fuzz: move MakeSwitch into init

.github/workflows/fuzz-nightly.yml

@@ -4,6 +4,10 @@ on:
   workflow_dispatch: # allow running workflow manually
   schedule:
     - cron: '0 3 * * *'
+  pull_request:
+    branches: [master]
+    paths:
+      - "test/fuzz/**/*.go"
 
 jobs:
   fuzz-nightly-test: