Future-proof testing for HTTP path creation and validation

A follow up to #2317

I wrote tests against `gapic-showcase`, but the coverage is underwhelming. It cannot verify that "first match wins" for example. Also, we (Rust folk) don't own the thing. Its RPCs could gain additional bindings at any time. We either have to loosen our test expectations or possibly deal with breaks when updating showcase.

For better coverage, we are adding tests in `gax-internal` against "once-generated-but-now-maintained-by-hand" code. Ideally this code should be owned by the generator.

To do this, we should factor out the `let builder = ...` code in `transport.rs` into a common template. Then we generate just that bit for our unit tests.

`src/wkt/tests/common/src/generated/.sidekick.toml` is an example of how this can work.