test_runner: warn if `only` is used without properly enabling only tests

[cjihrig]

What is the problem this feature will solve?

only tests require enabling only mode, otherwise only does nothing. This can lead people to think that only does not work.

What is the feature you are proposing to solve the problem?

The test runner should warn if someone tries to use only without properly enabling it.

What alternatives have you considered?

Doing nothing.

[richiemccoll]

@cjihrig I can take a look at this one, if nobody else has picked it up.

• Should this warning be a test diagnostic? or use a different mechanism for warnings?
• Would this be emitted once per parent test, once per test file or something different?

[MoLow]

@cjihrig I can take a look at this one, if nobody else has picked it up.

• Should this warning be a test diagnostic? or use a different mechanism for warnings?
• Would this be emitted once per parent test, once per test file or something different?

I think either process.emitWarning, or simply test.diagnostics is ok

[cjihrig]

I think we should prefer a test diagnostic.