Replace `docker-test` feature flags with better testing setup

> Separately, the `docker-tests` feature should not exist (IMO, like the `test` feature shouldn't) -- the most elegant way I've seen that navigated is discovering those features in `build.rs` and injecting a dynamic [`cfg`](https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-cfg) or ENV that lets the tests do what they need to do. I think the current code was an initial approach at this that should be removed.

_Originally posted by @vados-cosmonic in https://github.com/bytecodealliance/wasm-pkg-tools/issues/220#issuecomment-4866424022_

Start by inspecting build.rs and locating the current docker-tests and test feature declarations. Trace how the tests use those features, then determine whether a dynamic cfg or environment variable can provide the needed setup; done means the feature flags are removed while the tests still run correctly.

Written by the indexing model from the issue text.