Add tests for parseDurationString()

[wb9688]

• I carefully read the contribution guidelines and agree to them.
• I have tested the API against NewPipe.
• I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.

I added some tests for parseDurationString() and found out that apparently the days behavior was already broken, however YouTube doesn't seem to use days anymore.

[TobiGr]

@Redirion you approved this PR although you asked to include the changes from #345. I think you suggested changes are good. Is there a reason which I missed to not adopt them?

[wb9688]

@TobiGr: My personal reason would be that the function that calls it returns a long anyway, plus the speed difference is pretty much negligible I think.

[Redirion]

@TobiGr
I didn't notice the pr earlier (or more specific that it included a fix). It is good to merge.

@wb9688 think of 32bit arm entry level phones. Long arithmetics should be avoided if not needed. 100 negligible optimizations could make a feelable difference. :)

My PR did also change the return type.

[wb9688]

@Redirion: I meant e.g. https://github.com/TeamNewPipe/NewPipeExtractor/blob/dev/extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeStreamInfoItemExtractor.java#L102 and all the other functions calling it.

[Redirion]

Long arithmetics on a 32bit cpu require more ressources and operations. Storing and reading it as long should not be an issue.