Allow test-suite to detect trivial changes and not re-run tests

[danfinlay]

I don't know if this is a thing that anyone does yet, but there are some cases where we can plainly say tests don't need to run again, because they definitely will not affect our tests:

• README update
• CHANGELOG update

And some types of changes that don't need a full run of the suite, which may be detectable with static analysis:

• Lint fixes
• Code style fixes
• Some kinds of code organization?

Just opening an aspirational issue here to say "It'd be great if we didn't need to run our 20 minute test suite for every trivial change" :)

[danfinlay]

Note we can do this manually today: #9486

[whymarrh]

This should be pretty easy to configure—we pass a file pattern to mocha so we can ourselves ignore the commit if files matching that pattern haven't changed.