Improve tests on examples/rust/*

[neil2468]

Here are some opportunities...

1. The tests in examples/rust/get_started do not cover significant portions of code.
2. Some tests in examples/rust/* could still clash on TCP ports when run in parallel. Maybe they should ask the OS for free ports, rather then using hardcoded values (could use portpicker?).
3. example_test_helper could allow child processes to inherit environment variables from their parent. This might allow child cargo processes to use the mold linker in CI.
4. Maybe tests in examples/rust/* should use the the cargo --locked argument as is done in other parts of the Ockam code base (e.g. here).