rust-pyo3: reviving test-rust-revlog.py This test file was removed in 7346f93be7a4. Adaptation to the new `InnerRevlog` / `RustIndexProxy` structure was as easy as redefining the `parserustindex()` method to use `RustIndexProxy`. As we did before with `test-rust-ancestors.py`, we are preparing a mixin class that will contain tests for both bindings. Existing tests will migrate from `RustInnerRevlogTest` (the one for `hg-cpython`) to the mixin.