rpc/lib/client: jitter test updates and only to-be run on releases

* Updated code with feedback from @melekes, @ebuchman and @silasdavis.
* Added Makefile clause `release` to only run the test on seeing tag
`release` during releases i.e
```shell
make release
```
which will run the comprehensive and long integration-ish tests.

Makefile

@@ -35,6 +35,9 @@ test_race:
 test_integrations:
 	@bash ./test/test.sh
 
+release:
+	@go test -tags release $(PACKAGES)
+
 test100:
 	@for i in {1..100}; do make test; done