(E2E) testErrorChrome - Skip Backdrop and D8+ (much like D7)

(E2E) testErrorChrome - Skip Backdrop and D8+ (much like D7)

This test was added during 5.50.alpha (#23257, #22805).  It's purpose is to
assert that the page-chrome/site-wide-template is present on certain
error-messages; however, the underlying behavior was only implemented on
WordPress. It has not yet been implemented on other CMS.

Before
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* __WordPress__: Runs testErrorChrome. Expected to pass.
* __Drupal 7__: Skip testErrorChrome (mostly) because it is expected to fail.
* __Drupal 8+, Backdrop__: Runs testErrorChrome. Fails.

After
-----

* __WordPress__: Runs testErrorChrome. Expected to pass.
* ___Drupal 7, Drupal 8+, Backdrop__: Skip testErrorChrome (mostly) because it is expected to fail.

Comment
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It still runs `testErrorChrome` for `permission` errors; per MM discussion, it appears that this does actually work, but we need
to tune assertion for BD+D8.