doc: instructions for generating coverage reports
This PR adds instructions for generating coverage reports. I only added the instructions to BUILDING.md but not CONTRIBUTING.md because that defers to BUILDING.md in the Test section (See the BUILDING.md for more details.)
, but I'm happy to add the instructions to that document too if people think they're needed there.
Checklist
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make -j4 test
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documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc