Fix wrong "withTests" examples + add documentation

[renaudtertrais]

Issue:

What I did

Update addon-jest/README with good examples regarding the usage of withTests + add doc

How to test

Read README

[codecov]

Codecov Report

Merging #2398 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2398   +/-   ##
=======================================
  Coverage   19.92%   19.92%           
=======================================
  Files         293      293           
  Lines        6485     6485           
  Branches      758      772   +14     
=======================================
  Hits         1292     1292           
+ Misses       4600     4574   -26     
- Partials      593      619   +26

Impacted Files	Coverage Δ
lib/ui/src/modules/ui/configs/init_panels.js	25% <0%> (ø)	⬆️
addons/jest/src/hoc/provideJestResult.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/api/actions/api.js	50.47% <0%> (ø)	⬆️
lib/ui/src/libs/key_events.js	23.25% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Elements.js	0% <0%> (ø)	⬆️
lib/components/src/navigation/menu_link.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/element_check.js	41.17% <0%> (ø)	⬆️
addons/knobs/src/KnobManager.js	31.37% <0%> (ø)	⬆️
addons/knobs/src/components/PropForm.js	9.3% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Rules.js	0% <0%> (ø)	⬆️
... and 16 more

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[Hypnosphi]

LGTM. Was it the source of #2367?