Change Testingframework from mocha/chai to jest and add more tests

[Flui]

Expected Behavior

Write fast Unit- and Snapshottests with less configuration.

Current Behavior

For now there is a test-helper needed and

Possible Solution

install jest and refactor tests to work with jest

Context

This helps to provide almost 100% code coverage for testing. It is much easier to recognize if and in which way changes affect the rendered html.

[KamranAsif]

This isn't really a bug since there isn't anything wrong with mocha/chai.
That being said, feel free to open a PR switching to Jest and I'm sure the team will consider it

[wcjordan]

+1 to Kam's suggestion to open a PR. I don't think we need to switch frameworks, but adding some jest snapshot testing would be great. THanks