Rust: add integration tests for package host

Created by: olafurpg

Currently, our Rust package host is the only package host with no e2e tests. For other ecosystems, we have e2e tests here

https://github.com/sourcegraph/sourcegraph/blob/2230bfcf27fe4d34f67d741ea290ebec09ae4f31/dev/gqltest/search_test.go#L1394-L1434

The e2e integration tests are helpful because they have caught regressions that we fixed quickly in the PR https://github.com/sourcegraph/sourcegraph/pull/38652