Convert legacy tests to Functional tests

Hello,

Here is an attempt to convert the tests.

There are two remaining problems:
- I can't find the replacement for $this->drupalPostAjaxForm in modules/features_ui/tests/src/Functional/FeaturesUITest.php
- in modules/features_ui/tests/src/Functional/FeaturesCreateUITest.php near line 116:

 $this->assertEquals([
//      'excluded' => ['system.theme'],
      'required' => TRUE,
    ], $parsed_features_info);

I have commented the 'excluded' because I can't reproduce this behavior even outside the test.

Also thanks to @woprrr and @DuaelFr for their help.

Thanks for any help.

I forgot to change the test group because I changed its to have only one test triggered.

About the drupalPostAjaxForm, I found that drupalPostForm or submitForm should work but as there is no submit button when changing only the bundle select list it does not work.

Thanks for digging into this.

I have commented the 'excluded' because I can't reproduce this behavior even outside the test.

This may be related to #2860993: 'excluded' property not set in *.features.yml.