Expose URL of underlying provider

[frangio]

It can sometimes be useful to get the URL of the underlying provider of a Test Environment instance.

I decided to write an integration test in JavaScript using Test Environment to handle the provider lifetime, and I had to access the internals to get this information.

openzeppelin-sdk@0597939

const { provider } = require('@openzeppelin/test-environment');
// ...
  await provider.queue.onIdle();
  const url = provider.wrappedProvider.host;

For this specific use case it would be fine to provide an async function or getter returning the URL.

get url(): Promise<string>