Move test crates into a "testing" folder and add a ui (trybuild) test and ui-test helpers (#567)

* move test crates into a testing folder and add a ui test and helpers

* undo wee mixup with another PR

* cargo fmt

* clippy

* tidy ui-tests a little

* test different DispatchError types

* refactor dispatch error stuff

* name ui tests

* duff => useless

* align versions and cargo fmt

testing/ui-tests/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "ui-tests"
+version = "0.21.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+
+[dev-dependencies]
+trybuild = "1.0.63"
+scale-info = { version = "2.0.0", features = ["bit-vec"] }
+frame-metadata = "15.0.0"
+codec = { package = "parity-scale-codec", version = "3.0.0", default-features = false, features = ["derive", "full", "bit-vec"] }
+subxt = { path = "../../subxt" }