Use css style on component testing

[balanza]

Description

Some component tests rely on css class in order to determine the component properties. For example in 56a978b the toBeVisible() matcher look for style display: none to be applied to the element or one of its parents.

This PR proposes the <WithStyle> test utility. <WithStyle> is a higher-order component that prepends the application css to the component to be rendered. It's meant to be used only on tests that require such ability (for keeping the test footprint small).

In order to work the application css must be built already, thus a pretest script is added to package.json.
Furthermore, the package.json has been refactored a bit to make use of npm variables (not required, it can be reverted).

[balanza]

A quick thought on performance based on my laptop

# running command
time npm run test js/common/Tooltip/Tooltip.test.jsx

# without style
3.43s user 0.76s system 180% cpu 2.325 total

# with style, generated just in time (first run)
5.33s user 0.96s system 168% cpu 3.727 total

# with style, already generated
3.48s user 0.73s system 173% cpu 2.419 total

[balanza]

Addendum: with @jamie-suse we investigated a bit around the possibility to load CSS once for all JSDOM instances. It seems there is no this possibility: furthermore, it seems this feature is considered out-of-scope by JSDOM itself (see testing-library/jest-dom#70).

[nelsonkopliku]

LGTM