[Meta] Bring back all tests GREEN

Problem/Motivation

The 8.x-1.x branch's tests are broken. Tests, deprecation notices and coding standards need to be fixed to bring it GREEN again.

Proposed resolution

I've been working on it one submodule by one submodule this weekend and get all fixed. Just create a meta issue here to track progress.

Remaining tasks

Needs review.

Associated issues and targeted submodule(s)/module(s):

1. #3103583: Fix deprecation notices of render_example render_example
2. #3103569: Fix tests, coding standards and deprecation notices of stream_wrapper_example and file example stream_wrapper_example, file_example
3. #3103559: Fix deprecation notices for form_api_example form_api_example
4. #3103552: Fix ExamplesTest and deprecation notices, coding standards related in submodules cache_example, node_type_example, tablesort_example, testing_example, tour_example and examples
5. #3103548: Tidy up tablesort_example and tour_example tablesort_example, tour_example
6. #3103551: Tidy up testing_example testing_example
7. #3103550: Tidy up js_example #3103593: Rewrite the accordion example without using jquery_ui_accordion js_example
8. #3103518: Deprecation notices: Drupal\Tests\BrowserTestBase::$defaultTheme is required in drupal:9.0.0 batch_example, block_example, config_entity_example, content_entity_example, cron_example, dbtng_example, email_example, events_example, field_example, hooks_example, menu_example, page_example, pager_example, plugin_type_example, queue_example, session_example, tabledrag_example
9. #3103521: Tidy up and fix ajax_example ajax_example
10. #3072441: Tidy up and fix phpunit_example phpunit_example

And one suggestion [no patch]: #3103520: Move all examples/submodules into the modules folder

And BTW, the patches from #3103548: Tidy up tablesort_example and tour_example and #3103551: Tidy up testing_example are included in #3103552: Fix ExamplesTest and deprecation notices, coding standards related in submodules