[DX]: Document running test suite for all workspaces

[pieterocp]

Description

As a developer who hasn't worked on many npm projects using workspaces, I wasn't aware how npm has the ability to run a command over all the workspaces . This means with this one-liner, a developer can easily run nearly all of the test suites in one go.

So off the back of this, this PR changes the contributing documentation slightly to make it clearer it's possible to run the tests over the whole project rather than having to run them one by one.

Note, once I get a thumbs up emoji/an affirmative comment that is a welcomed change, then I'll make the change to all the different languages using auto-translation tools to amend the # run tests over all workspaces comments into respective languages.

Contribution Checklist:

• The pull request only addresses one issue or adds one feature.
• The pull request does not introduce any breaking changes
• I have added screenshots or gifs to help explain the change if applicable.
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

[helloanoop]

Looks good. @pieterocp !