Use tokio::time::advance in the genesis rate-limit test

[teor2345]

Motivation

In #2255, we rate-limited initial genesis block downloads, and added a test for the rate limit.

But we can make the test faster using tokio::time::advance. This speed up will let us test multiple retries.

API Documentation

https://docs.rs/tokio/1.6.1/tokio/time/fn.advance.html

Solution

• Use pause and advance to change tokio's internal time in the request_genesis_is_rate_limited test
• Increase the number of retries in the test

Alternatives

We could leave the test as it is, and accept the slower time. (And smaller coverage.)

Related Work

We also want to replace the atomics in the test #2268

[teor2345]

This cleanup is not required.