Fix borderRadius-related bug in [ButtonGroup]

[yujonglee]

Description

1. See Top-Right and Botton-Right radius. It is not working properly when data size is 1.
   So I write tests, fix bug, and refactor code.
   (remove chained ternaries as mentioned in Remove some chained ternaries #45)
   

2. In the process of fixing bug, I made trivial changes.

• Move CHILD_index testID to make testing easier.
• Remove useless theme prop / default value
• Remove testID prop
  I don't think user need testID prop. It is applied to Outermost View wrapper. But why user need to get that? They might write test code relate to each button. But they can to that with getByText or element_index testID. So it is no need to give them free to name it. Just name 'container' is fine for development use.
• Rename testID ([userInput - CHILD] -> [container - element])
  I think container - element gives better abstraction.
• Data prop shouldn't be optional. I remove default value and make it required.

3. I use givens library. I will explain reasons in discussion soon.
   Although it supports Typescript, I didn't define types because I think It's bit much for test code.

import getGiven from 'givens';
interface myVars {
  var1: number; // the keys you intend to use
  var2: string; // and their types
}
const given = getGiven<myVars>();

Related Issues

#45

Tests

I add tests to ensure [ButtonGroup] calculates proper borderWidth and borderRadius.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• Run yarn test or yarn test -u if you need to update snapshot.
• Run yarn lint
• I am willing to follow-up on review comments in a timely manner.

[codecov]

Codecov Report

Merging #62 (5a10afb) into master (63175fd) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   93.94%   94.22%   +0.27%     
==========================================
  Files          21       21              
  Lines         545      554       +9     
  Branches      236      235       -1     
==========================================
+ Hits          512      522      +10     
+ Misses         33       32       -1     

[yujonglee]

I should have split trival changes to different PR. 😅

At least props change like theme and testID shoud split to other PR to make changes clear. (And they need docs updates too)

Edit: I did it.

[hyochan]

You've rocked!! 👊