RichText: Fix intermittent test failure

[ellatrix]

Description

After #16875, this fixes one of the new e2e tests that sometimes fails.

The problem is that we don't wait for the next animation frame:

gutenberg/packages/rich-text/src/component/index.js

Line 308 in 005e030

this.rafId = window.requestAnimationFrame( this.onSelectionChange );

How has this been tested?

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[ellatrix]

Going to merge this as it's just a test change without any other code changes.