wasm-bindgen-test-runner drops path from webdriver remote URL

[cdata]

Describe the Bug

When making use of the remote webdriver URL for wasm-bindgen-test-runner (e.g., the one configured by setting the CHROMEDRIVER_REMOTE environment variable), wasm-bindgen-test-runner drops the path component of the URL, replacing it entirely with /session. This prevents correct use of the test runner when targeting some remote webdrivers.

For example, Browserstack's webdriver service endpoint is available on https://hub-cloud.browserstack.com/wd/hub/, but wasm-bindgen-test-runner truncates the URL to https://hub-cloud.browserstack.com/session before sending a request to create a session.

Steps to Reproduce

1. Expose a webdriver service endpoint on a sub-path of a domain (e.g., http://example.com/webdriver)
2. Set CHROMEDRIVER_REMOTE=http://example.com/webdriver/ (note: the trailing slash is also relevant to this topic)
3. Run your browser tests that make use of wasm-bindgen-test-runner

Expected Behavior

A session is created and tests are able to be run.

Actual Behavior

A confused webdriver that doesn't start the session. In my case, I was seeing a response code 404 and the body:

{"value":{"message":"Session not started or terminated"},"sessionId":"","status":13}

Additional Context

After a lot of debugging, I was able to reduce the issue down to the fact that Url::join interprets a leading / as an indication that the input path fragment is absolute with respect to the origin. So, when Url::join is invoked with the intention of appending /session to the configured URL, it drops the path of that URL.

There are several places in headless.rs where Url::join is used, for example: https://github.com/rustwasm/wasm-bindgen/blob/7993d911f2c15ec52a8068fc1701625ad2757d76/crates/cli/src/bin/wasm-bindgen-test-runner/headless.rs#L519

I patched wasm-bindgen here to test out a fix and was able to unblock myself: https://github.com/rustwasm/wasm-bindgen/compare/main...cdata:main

The fix in my patch amounted to removing the leading / from all paths that previously started /session. There may be alternative approaches to fix this.