Change path to mimemail_example form

Currently this form is at /example/mimemail_example, which is the same path that was used in Drupal 7.

However, #3128807: Add tests for the mimemail_example module demonstrates conclusively that the DrupalCI environment will not work if we use a path starting with /example (it's not exactly a DrupalCI problem, but that will be addressed in the core issue queue).

So in order to proceed with adding tests to mimemail_example we need to choose a new path that won't conflict with the testbot environment.

Using /admin/config/system/mimemail/mimemail_example seems reasonable to me. Putting it under /admin is more consistent with how we usually do things in Drupal and allows the form to inherit admin access permissions.

I also added a configure link to mimemail_example.info.yml so this form can be found. And since this form is located "under" the Mime Mail settings page, I added tabs to the Mime Mail settings page so that the user can easily switch between the settings and the mimemail_example Test page. This also makes the Test page easy to find.