Testing: Add Unit Test coverage to core functions

[chrisli30]

From Brendan:

I suggest that for starters, any parts of rWallet that are not UI - i.e. do not touch react or react-native APIs - let's follow this process:

1. identify which parts fall into this category
2. refactor to extract them into their own spearate files or dirs within the project (applies only where these parts are within the UI code)
3. write tests targeting these parts
4. modularise by refactoring to extract these parts into a separate module (applies only where these parts are not already in their own module)

We need:

1. First to set up the framework for testing pure Javascript code. We can use Mocha.
2. Add test cases to utils functions in src/common

[bguiz]

s/mocha/jest/

because: (1) more react-specific test helpers available (for future), and (2) built-in coverage