fix(material-experimental/mdc-radio): de-duplicate test harness logic

[crisbeto]

Changes the MDC-based radio harnesses to extend the base ones directly since the logic is identical, aside from some selectors.

[jelbourn]

LGTM

[mmalerba]

I'm seeing the following error from this (and some of the other PRs to share harness code):

Type 'import("material/radio/testing/radio-harness").MatRadioButtonHarness[]' is not assignable to type 'import("material-experimental/mdc-radio/testing/radio-harness").MatRadioButtonHarness[]'.
  Type 'import("material/radio/testing/radio-harness").MatRadioButtonHarness' is not assignable to type 'import("material-experimental/mdc-radio/testing/radio-harness").MatRadioButtonHarness'.
    Property '_textLabel' is protected but type 'MatRadioButtonHarness' is not a class derived from 'MatRadioButtonHarness'.

The code that causes it looks like this:

import {MatRadioButtonHarness, MatRadioGroupHarness} from '@angular/material/mdc-radio/testing';

...

let radioGroupHarness: MatRadioGroupHarness[];
let radioButtonRelations: MatRadioButtonHarness[];

...

radioGroupHarness =
    await harnessLoader.getAllHarnesses(MatRadioGroupHarness);
radioButtonRelations = await radioGroupHarness[0].getRadioButtons();

[crisbeto]

It's odd that our own build didn't complain about it. Maybe we can flip on a compiler flag to make it behave closer to g3? Also is it possible that they're importing both the non-MDC and MDC harnesses and they're using one to type the variable and the other one to look up the elements?

[mmalerba]

That's what I assumed was happening at first, but no - they're not importing anything from radio/testing and the variables are explicitly typed with the class imported from mdc-radio/testing like I showed above.

[mmalerba]

Maybe it has something to do with the with function? this looks a little suspect:

return new HarnessPredicate<NonMdcMatRadioButtonHarness>(MatRadioButtonHarness, options)

[crisbeto]

I see, I managed to reproduce it locally too. It's only the second call that is causing the compilation error. I think that it'll have to use the same approach where we have a base class for the harnesses. I'll rework it and let you know when it's done.

[crisbeto]

Reworked the PR to avoid the compilation error.

[andrewseguin]

Passes TGP - will be merged on the next round

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