Finish translation of testing

Done translation of `testing.md`. Tried my best not to introduce too many "literal" word-by-word translations of phrases that may divert from the original "dev-related" meaning. However, two specials cases are 

- `Assertions`  which I translate to စစ္ေဆးျခင္း and
- `Mock` to အတုျပဳလုပ္

A bit awkward but they are first accompanied with respective original english words when I first introduce them.

> အတုျပဳလုပ္ (mock) 
> Framework Assertions (Framework စစ္ေဆးျခင္းမ်ား)

It kinda works, I guess.