Node core building instructions in one place

[trivikr]

Is your feature request related to a problem? Please describe.
Many Code and Learn participants faced problem to find the documentation for running single tests:

• It's documented under running tests in Pull requests
• It's not documents in BUILDING.md

Describe the solution you'd like
Remove duplicate documentation and have single location where build instructions are defined

[fastereder]

i´m working on this

[trivikr]

Reopening as the commit 5a30621 just adds instructions to run single test in BUILDING.md

The ask is to have instructions in single place - instead of repeating them in Pull Requests, BUILDING.md and TESTING.md