Add $defaultTheme property if missing on BrowserTestBase

[mglaman]

Description

Add protected $defaultTheme = 'stark'; to functional tests if missing.

This could cause test failures because they assume on markup for Classy, which
the theme should be changed to starterkit as Classy is going away in Drupal 10.

To Test

Drupal.org issue

https://www.drupal.org/project/rector/issues/3299835

[mglaman]

It's running, but not making changes:

2022-08-10T16:07:45.9590491Z [file] web/modules/custom/rector_examples/test/src/Functional/BrowserTestBaseGetMock.php
2022-08-10T16:07:45.9590707Z [rule] DrupalRector\Rector\Class_\FunctionalTestDefaultThemePropertyRector

And here:

2022-08-10T16:07:44.9331621Z [file] web/modules/custom/rector_examples/tests/src/Functional/PublicStaticModulesTest.php
2022-08-10T16:07:44.9332120Z [file] web/modules/custom/rector_examples/tests/src/Functional/AssertFieldByIdTest.php
2022-08-10T16:07:44.9332384Z [rule] DrupalRector\Rector\Class_\FunctionalTestDefaultThemePropertyRector
2022-08-10T16:07:44.9332594Z [rule] DrupalRector\Rector\Deprecation\AssertElementPresentRector

[agentrickard]

@mglaman This one has a merge conflict I am uncertain about

[bbrala]

This was one of the top issues last bot run. So thought I'd update and fix it. All seems fine now.

[mglaman]

I approve of the changes 👍 but it's my own PR so, this is my +1 for someone to approve and merge

[bbrala]

As per matt, approved