Test that Header label + customized label wrapper of a Views table display is not double escaped

Issue summary update:

The double-escaping in this issue was fixed in #2502089: Remove SafeMarkup::set() in template_preprocess_views_view_table(). However, that patch did not add a test for double-escaping this element. This issue has a test-only patch to address this.

Problem/Motivation

While reviewing #2502089: Remove SafeMarkup::set() in template_preprocess_views_view_table() it was discovered that customizing a label wrapper in a view that is being displayed as a table results in the double-escaping of the label in the table header.

Steps to reproduce

1. Install Drupal 8
2. Edit the view for admin/content and customize the label wrapper for one of the fields
   
3. Visit admin/content and observe that the label is not being correctly rendered
   

(YesCT's screenshots shamelessly stolen from the previous issue)

Remaining tasks

Fix it.

User interface changes

n/a

API changes

n/a