Ensure metadata is in sync with running node during tests (#333)

* add test-runtime crate to dynamically obtain metadata/node_runtime for tests

* cargo fmt

* Download substrate prior to cargo calls (hopefully)

* add README explaining test-runtime

* Fix CI, fmt and clippy

* more clippy

* tweak test-node readme

* fmt the clippied

* A little tidy up in build.rs

* use ureq and raw string

* Don't export unnecessary metadata

* async_std/RpcClient/bytes instead of ureq/Value/hex

* newline

* document try_from_url error unwrap

test-runtime/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "test-runtime"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+subxt = { path = ".." }
+sp-runtime = { package = "sp-runtime", git = "https://github.com/paritytech/substrate/", branch = "master" }
+codec = { package = "parity-scale-codec", version = "2", default-features = false, features = ["derive", "full", "bit-vec"] }
+
+[build-dependencies]
+subxt = { path = ".." }
+async-std = { version = "1.9.0", features = ["attributes"] }
+sp-core = { package = "sp-core", git = "https://github.com/paritytech/substrate/", branch = "master" }