Use clock library to enhance testing

[Bolodya1997]

Issue

We are running all our unit tests on multiple platforms with --race flag. It does mean that any timeout test becomes either slow or not stable or both, because we have totally no idea how long could it take for something to happen.

Solution

Instead of using timeouts in tests we can use some clock library (e.g. https://github.com/benbjohnson/clock) to use mocked time in tests and real time in applications.

[denis-tingaikin]

@edwarnicke Can we go with this?

This can speed up our unit tests. This problem is most actual for #650 where we have long tests to check functionality based on timers.

[edwarnicke]

@denis-tingaikin I had a long talk with @Bolodya1997 about this... and am fully in support :)

[Bolodya1997]

Linked PRs:

• add clock package - Add Clock #782
• rework pkg/networkservice to clock - Rework pkg/networkservice to use clock.Clock #783
• rework pkg/registry to clock - Rework pkg/registry to use clock.Clock #784
• rework sandbox to clock - [sdk#764] Rework sandbox to use clock.Clock #785

[Bolodya1997]

Blocker for #823.

[edwarnicke]

@Bolodya1997 Is this still in progress?

[Bolodya1997]

It is actually done (#785) but blocked by #811. Just need to merge dependent PR and rebase.