Add functional test coverage for placing multiple blocks

Proposed commit message:

Issue #1874822 by Cottser, porchlight, lbainbridge, LittleCoding, mojzis | xjm: Add functional test coverage for placing multiple blocks.

Problem/Motivation

This is a followup issue for #1535868: Convert all blocks into plugins →

• #1535868: Convert all blocks into plugins → introduces the ability to place multiple instances of the same block within the same theme, but does not add any test coverage for this new functionality.

Proposed resolution

Add functional test coverage for placing multiple instances of the same block, including:

• Placing the same block twice.
• Placing the same block with different settings.
• Coverage for multiple menu blocks.
• Coverage for multiple custom blocks.
• Coverage for multiple Views blocks.

Remaining tasks

Related issues

• #1874498: Provide and use API methods for retrieving base plugin and derivative names from block plugin IDs →