Improve the test section of the CONTRIBUTRING file.

[Elhebert]

What does it do?

This PR adds more clarity in how to run the e2e test suite. It also document how to run the tests only for CE or how to add a license to run the tests in EE.

Why is it needed?

The current CONTRIBUTING guide isn't very explicit on how to run the e2e tests suite, which can cause some issues as the information is hard to find in the code base.

How to test it?

N/A

Related issue(s)/PR(s)

Let us know if this is related to any issue/pull request

[codecov]

Codecov Report

Merging #10848 (ab9221a) into master (8972d78) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10848   +/-   ##
=======================================
  Coverage   58.12%   58.12%           
=======================================
  Files         185      185           
  Lines        6429     6429           
  Branches     1395     1395           
=======================================
  Hits         3737     3737           
  Misses       2230     2230           
  Partials      462      462           

Flag	Coverage Δ
front	?
unit	58.12% <ø> (ø)

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[oiorain]

Nice work. LGTM!