Move workspace test into e2e format

NIT: I really wonder if these should be fixtures. Feels wasteful to add like 10 new folders, but also feels unfortunate to write temp files in the unit tests.

_Originally posted by @vados-cosmonic in https://github.com/bytecodealliance/wasm-pkg-tools/pull/231#discussion_r3572392959_


Mimic `#[cargo_test]` e2es:
https://github.com/rust-lang/cargo/blob/a595d0da21f228b7fdae64d3d5c0e527ea66bb59/tests/testsuite/workspaces.rs#L2867-L2932

Locate the current workspace test and compare it with Cargo's #[cargo_test] examples in tests/testsuite/workspaces.rs, especially lines 2867-2932. Determine how the test can follow that e2e format rather than writing temporary files in unit tests; done means the workspace behavior is covered by an e2e-style test without the discussed fixture or temporary-file drawback.

Written by the indexing model from the issue text.