use a mock clock in bandwidth counter tests

[marten-seemann]

These tests are currently flaky, and we won't be able to proceed with repo consolidation (libp2p/go-libp2p#1556) until they're fixed. Injecting a mock clock (similar to what we did in https://github.com/libp2p/go-libp2p/pull/1516/files) should be a straightforward way to test this reliably, and dramatically speed up the test at the same time.

[schomatis]

@marten-seemann I suspect I'm missing some context here (I'm new in libp2p and the term 'bandwidth counter' doesn't mean anything to me) and will need this laid out in a bit more detail:

These tests are currently flaky

Which tests?

Injecting a mock clock

Where?

[marten-seemann]

Which tests?

The bandwidth tests.

Where?

Into the metrics/BandwidthCounter. It’s using the flow metrics repo, so probably we need to do it there too. Basically, the goal is that we don’t use any calls to time.Now() any more, but only to a clock.Now(). When run in production, clock.Now() use time.Now(), but in tests we can use a mock clock.

[schomatis]

Sounds good, working on this.

[schomatis]

Upstream work in libp2p/go-flow-metrics#18. After that lands will proceed with applying the mock clock here.