docs: match CONTRIBUTING.md's test command to what CI runs (#8427)

`cargo test --workspace ...` segfaults on rustpython-capi when run from
the workspace root; it needs its own config that only applies inside
crates/capi. CI already excludes it from the workspace run and tests it
separately. Document both.

Closes #8415

Assisted-by: Claude Code:claude-sonnet-5

CONTRIBUTING.md

@@ -89,7 +89,15 @@ $ pytest -v
 Rust unit tests can be run with `cargo`:
 
 ```shell
-$ cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher
+$ cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capi
+```
+
+`rustpython-capi` needs to be tested from inside its own directory, since it has a
+separate `cargo` config that only applies there:
+
+```shell
+$ cd crates/capi
+$ cargo test
 ```
 
 Python unit tests can be run by compiling RustPython and running the test module: