part 2: Add basic tests for insertParagraph in `contenteditable=plaintext-only
The behavior should be same as when contenteditable=true. Therefore,
this test checks only the basic behavior.
Differential Revision: https://phabricator.services.mozilla.com/D224186
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1921162
gecko-commit: 3856ef423b913fce226ff04059fb818036ab9d36
gecko-reviewers: m_kato