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

tests/integration/main.rs

@@ -15,7 +15,6 @@
 // along with subxt.  If not, see <http://www.gnu.org/licenses/>.
 
 mod codegen;
-mod runtime;
 mod utils;
 
 #[cfg(test)]
@@ -23,5 +22,5 @@ mod client;
 #[cfg(test)]
 mod frame;
 
-pub use runtime::node_runtime;
+pub use test_runtime::node_runtime;
 pub use utils::*;