Spawn test node process for integration tests (#252)

* Move tests to module in own file

* Add test-node-proc crate

* Move test node to main lib, fix compilation errors

* Fmt

* Attempted port scanning

* Use global static port to allocate open ports

* Register missing type sizes

* Fix port scanning and balances test

* Don't wait for blocks on insert_jey test

* Fmt

* Remove unused dependencies

* Download v3.0.0 rust binary

* Move substrate bin to home dir

* Add test instructions to README

* Figure out the $PATH

* Remove path echo

* Add check for missing substrate binary on the PATH

* @niklasad1 review: don't include client feature for tests

README.md

@@ -6,6 +6,28 @@ A library to **sub**mit e**xt**rinsics to a [substrate](https://github.com/parit
 
 See [examples](./examples).
 
+## Integration Testing
+
+Most tests require a running substrate node to communicate with. This is done by spawning an instance of the
+substrate node per test. It requires an executable binary `substrate` at [`v3.0.0`](https://github.com/paritytech/substrate/releases/tag/v3.0.0) on your path.
+
+This can be done by downloading the prebuilt binary:
+
+```bash
+curl "https://releases.parity.io/substrate/x86_64-debian:stretch/v3.0.0/substrate/substrate" --output substrate --location
+chmod +x ./substrate
+mv substrate ~/.local/bin
+```
+
+Or installed from source via cargo:
+
+```bash
+cargo install --git https://github.com/paritytech/substrate node-cli --tag=v3.0.0 --force
+```
+
+
+
+
 **Alternatives**
 
 [substrate-api-client](https://github.com/scs/substrate-api-client) provides similar functionality.