Make tests under editing/plaintext-only not allow to pass with <br> elements when white-space is pre, pre-wrap or `pre-line
Currently, the expectations include <br> element cases even if white-space
value is pre, pre-wrap or pre-line. However, in these cases, only \n
should be allowed because <br> elements are not converted to \n if web apps
gets the editing host value with .textContent.
Differential Revision: https://phabricator.services.mozilla.com/D226441
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1926194
gecko-commit: 69bbad8c218d32f119ad2a87445ef1f067a7426c
gecko-reviewers: m_kato