Improve switch to draft button e2e test robustness

[chad1008]

Description

Follow up to #37491

This PR improves the XPath selector for the post scheduling calendar as discussed in #38717 (review) to add more specificity and make the test more robust.

cc @ciampo

Testing Instructions

all tests should pass

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.