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

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = [".", "cli", "codegen", "macro"]
+members = [".", "cli", "codegen", "macro", "test-runtime"]
 
 [package]
 name = "subxt"
@@ -48,6 +48,7 @@ env_logger = "0.8.3"
 tempdir = "0.3.7"
 wabt = "0.10.0"
 which = "4.0.2"
+test-runtime = { path = "test-runtime" }
 
 sp-keyring = { package = "sp-keyring", git = "https://github.com/paritytech/substrate/", branch = "master" }