Fix openDocumentSettingsPanel and tests

[cadic]

Description of the Change

This PR fixes multiple issues with openDocumentSettingsPanel and openDocumentSettingsSidebar commands:

• Close welcome guide action failed in older WordPress versions, fixed in Close Welcome Guide #42
• In WordPress prior to 5.4 the Document settings tab is called "Document", in later core versions it is renamed to the current post type name (Post, Page, etc.)
• In WordPress prior to 5.4 panel names were called with capitalized style. Starting from 5.4 panel names are called with Upper-case-first style.

Closes #36

Tested against every WordPress core major release from the minimum 5.2 to the latest nightly build.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

[cadic]

Some tabs (Tags, Categories, etc.) does not exist after the DOM is ready. They are added a few moments later after REST calls. In this case, using cy.get('button').contains(name) doesn't work: cy.get() is called before the tabs appear in DOM, finds other panels, but fails to filter any containing the name.

Using new panelButtonSelector with button:contains("tab name") to wait for the elements to appear in DOM.

[iamdharmesh]

@cadic Thanks for fixing this.
LGTM!! 👍