Add test coverage collection script

[jryans]

This makes it clear to how collect basic test coverage when desired.

See also matrix-org/matrix-js-sdk#1677

[germain-gg]

Looks good to me 💯 !

I just ran the tests with the --coverage flag, and it looks like the coverage/ folder is not listed in .gitignore. It would probably be sensible to add it there to avoid accidental commits

[jryans]

I just ran the tests with the --coverage flag, and it looks like the coverage/ folder is not listed in .gitignore. It would probably be sensible to add it there to avoid accidental commits

So far, I have not done so, because the reporter config here does not make a directory, but sure, could do.