Add toHaveClass custom matcher (closes #2)

[gnapse]

As discussed in #2 (closes #2)

One thing to note about it: I did this custom matcher so that it can even be used to match more than one class at the same time:

// <button data-testid="delete-button" class="btn extra btn-danger">
//   Delete item
// </button>

// This passes, even though `"btn btn-danger"` are not contiguous in the actual class attr
expect(getByTestId('delete-button')).toHaveClass('btn btn-danger')

I think this makes it much more useful, and was not difficult to support.

[codecov]

Codecov Report

Merging #4 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #4   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          85     95   +10     
  Branches       24     26    +2     
=====================================
+ Hits           85     95   +10

Impacted Files	Coverage Δ
src/jest-extensions.js	100% <100%> (ø)	⬆️
src/extend-expect.js	100% <100%> (ø)	⬆️

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[antsmartian]

@gnapse You are fond of jest custom matchers :) Looks cool though!

[gnapse]

Yeah, I'd like to strike some balance between usefulness and avoiding bloating this lib, so I don't really think I'll propose any new ones for the time being, but these two (.toHaveAttribute and now .toHaveClass) came up from tests I was writing myself with react-testing-library, and I thought that testing for these two things is really common. Plus they fit very nicely in the context of testing the dom, which is what this new library extracted from react-testing-library is for. That was a nice move!

[kentcdodds]

This is great!