yarn test: Default to bleeding edge configuration (#21238)

My personal workflow is to develop against the www-modern release
channel, with the variant flags enabled, because it encompasses the
largest set of features. Then I rely on CI to run the tests against
all the other configurations.

So in practice, I almost always run

```
yarn test -r=www-modern --variant TEST_FILE
```

instead of

```
yarn test TEST_FILE
```

So, I've updated the `yarn test` command to use those options
by default.