Simplify #[test] generation for *.wast files (#507)

This commit simplifies the build script slightly for generating tests by
doing a few dull refactorings:

* Leaves formatting to `rustfmt`
* Extract bulk of code execution into a top-level shared `run_wast`
  function so each test is a one-liner
* Use `anyhow` for errors both in the script and in tests

Cargo.toml

@@ -38,6 +38,9 @@ libc = "0.2.60"
 rayon = "1.1"
 wasm-webidl-bindings = "0.6"
 
+[build-dependencies]
+anyhow = "1.0.19"
+
 [workspace]
 members = [
   "misc/wasmtime-rust",