Make Localization.relativeTime testable

[XiangRongLin]

What is it?

• Codebase improvement (dev facing)

Description of the changes in your PR

• Make Localization.relativeTime testable.
• Add some simple tests

Context

• Problem is global state in static variable prettyTime. But for performance reasons on Android that is preferred.
• At first i wanted to make a manager class which would then not be static. similiar how i did Extract export database logic into own class #5059 and Extract settings import #5225. But this would come with a performance loss, because every call to relativeTime would then create a manager object, create prettyTime, determine the app language, etc. https://developer.android.com/training/articles/perf-tips#ObjectCreation
• This solution allows injecting prettyTime for tests while only adding a method call when initializing prettyTime and otherwise keeping the performance fo relativeTime

APK testing

https://github.com/TeamNewPipe/NewPipe/actions/runs/466273618

Due diligence

• I read the contribution guidelines.

[Redirion]

Maybe I am just unable to understand Kotlin, in this case please enlighten me. But this assertion will never be true because both times are just 5 days apart from each other - especially when I compare it with the last test? So shouldn't it be assertEquals("5 days from now", actual) here as well?

[XiangRongLin]

You are absolutly correct, but this is actually what pretty time returns. I would guess it's either a bug in PrettyTime or a limitation when comparing with GregorianCalendar.

For these tests i went with the assumption that the current state is correct. So i just added assertions with what is returned, so future regressions are covered.

[Redirion]

okay, can you add this explanation as an inline comment so that no one stumbles upon this later?

[XiangRongLin]

Do you mean the first paragraph of the second (or both)?

I would add the first with the note that further research is required

[Redirion]

just add anything meaningful like // yes this assertion is true, even if it should be 5 days, it works as it is. Future research required

Maybe it has to do something with timezone shift. Could it be that Locale.ENGLISH is defaulting to the global standard locale US in Android? This would explain why 2021, 1, 1 is in fact 2020,12,12 normalized to UTC and could explain "1 month from now"

[Redirion]

sigh did just see #5230

mayb we should have merged this earlier