Benchmark the start up time for each shim

[Mossaka]

Currently, we benchmark when the task_start is invoked to the end of the execution of the wasm module.

fn run_wasmtime_test_with_spec(wasmbytes: &[u8]) -> Result<u32, Error> {
    let (exit_code, _, _) = WasiTest::<WasmtimeTestInstance>::builder()?
        .with_wasm(wasmbytes)?
        .build()?
        .start()?
        .wait(Duration::from_secs(10))?;
    Ok(exit_code)
}

We would love to benchmark the start up of the shim, namely from the entrypoint of shim_main() to run_wasi().

[dblnz]

I'm giving this a try