Add signal rate limiting and automatic tests

[szarykott]

Rationale

OOMHero would send a signal to a process every second. This could overload some systems. I want to add a cooldown feature to limit how often signals are sent. It was inspired by work by @rtclauss on one of the forks, I cherry-picked his commit and based my work on it, therefore he is visible in the commit history.

Functionality-wise this change is backward compatible - pulling a new version of OOMHero would not change the observed behavior of it if an old configuration was used, due to the cooldown default being 1 second.

What this PR contains

• Adding the cooldown feature itself
• Adding a cooldown feature made the code quite difficult to read. To make the code read better I introduced "state machine" with methods on it that have meaningful names, therefore making core logic easier to follow
• The logic itself became less trivial and I was worried that what I implemented might not be correct, therefore I added automatic tests. To do so it required:
  • abstracting os.Process behind proc.Process interface for the main application and creating mock implementation TestProcesses for tests
  • adding a dependency on a new package testify to have nice-to-read asserts
  • changing the structure of the main loop in order to be able to in tests:
  • inject TestProcesses
  • "control time" so the tests do not take minutes to run
• Adding documentation of new feature

[rtclauss]

Awesome! I look forward to trying this new feature out.

[szarykott]

@ricardomaraschini I also added one more commit regarding "time management" - it uses time that comes from the ticker as now. This makes the tests more readable and resemble the real application more as in both cases "now" comes from the same source code-wise. Please review.

[ricardomaraschini]

Merged, excellent work.

[szarykott]

I still did not implement parsing cooldown as 1m30s! This was a great idea, I just did not have time to implement it last week. Do you want me to submit another pull request?

[ricardomaraschini]

@szarykott Absolutely!