Expand Javascript Functional Tests

In #2587531: Convert Automated Tests to BrowserTestBase JavaScript Functional Tests were added to Extlink with a basic example which checks to see if an external link and mail to link were tagged as expected.

These tests need to be expanded to:

• svg with href #2855615: External Links not working after an href inside an SVG...
• with/without extlink
• test extlink class='0' #2821803: Missing variables and variable type mismatches
• with/without mailto
• alert - warning box, postponed pending understanding of handling alerts in javascript functional tests #2939078: Cancel event in the external page disclaimer is not working properly?
• target/target no override
• Test #1672650: placing icon BEFORE link, not after
• regex exclude
• regex include
• css exclude
• css include
• no follow #2870492: Allow rel="follow" to not be overridden
• subdomains
• Test different domain types with regards to our regular expressions #2914838: (Sub-)domain detection not reliable..
• Add tests for administration form, setting each variable and saving.

Example of changing config:
https://api.drupal.org/api/drupal/core%21modules%21color%21tests%21src%2...

Example of link clicking:
https://www.drupal.org/docs/8/phpunit/phpunit-javascript-testing-tutorial