adds command for the codegen tests (#1310)

Co-authored-by: Elmer Bulthuis <elmerbulthuis@gmail.com>

Author: elmerbulthuis

tests/README.md

@@ -55,6 +55,17 @@ Use `git submodule init` +  `git submodule update` before running the tests.
 
 The tests are generated by a macro `codegen_tests` in [crates/test-helpers](../crates/test-helpers/).
 
+You can run all `codegen` tests in rust with the following command:
+
+```sh
+cargo run test \
+  --languages rust  \
+  --artifacts target/artifacts \
+  --rust-wit-bindgen-path ./crates/guest-rust \
+  tests/codegen
+```
+
+
 ## Testing wit-bindgen - `runtime`
 
 Otherwise tests are organized in `tests/runtime/*`. Inside this directory is a