Fix Eventually and EventuallyWithT to always run condition

[brackendawson]

Summary

If Eventually was called with a tick larger than waitFor, or a tick very slightly smaller than waitFor, or very small tick and waitFor values, then a timing race can occur where the select statement falls into the waitFor case first and the condition function is never run.

Changes

• Add a timeout to the unit test so that a deadlock is found after 1 second rather than 600 seconds.
• Swap the order that the ticker and timer are created so that in the case that tick is very slightly smaller than waitFor, then the timer will never yield before the ticker.
• Run condition once before the loop and initialise tick in the for loop to (<-chan time.Time)(nil) so that we always run condition statically, then check its result, then run condition based on ticks.

Motivation

Having a timing race where it is possible for condition to never be run causes our CI tests to flake, and is not how I think the user would expect Eventually can behave.

Related issues

Closes #1652

[brackendawson]

This might be a dupe of #1427

[brackendawson]

The same fix is needed in Never.

[brackendawson]

#1657 is a far better approach I think.