Add instructions to run gulp build before test

[piotr-cz]

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Issue or Feature Request Description:
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GitHub PR template says

ensure that gulp lint test passes locally.

However it doesn't say that for gulp test it's required to install packages/* dependencies via gulp build command.
Also in the CONTRIBUTING.md file there are no such instructions.

I suggest to rephrase it one of these:

• ensure that gulp build && gulp lint test passes locally.
• ensure that gulp build gulp lint test --series passes locally.
• ensure that gulp lint test --series passes locally (run gulp build to install dependencies first).