Refactor mgmt of misc testsuite (#101)

Changes:
* use [tempfile] crate for auto mgmt of temp dirs
* use concrete types in place of generics in `utils` module

[tempfile]: https://github.com/Stebalien/tempfile

Cargo.toml

@@ -37,6 +37,7 @@ cranelift-wasm = "0.41.0"
 cranelift-native = "0.41.0"
 target-lexicon = "0.4.0"
 pretty_env_logger = "0.3.0"
+tempfile = "3.1.0"
 
 [patch."https://github.com/CraneStation/wasi-common"]
 wasi-common = { path = "." }