Add explicit test coverage for the BlockPluginUI

Posted by xjm

Problem/Motivation

This is a followup issue for #1535868: Convert all blocks into plugins and #1874830: Add a test block plugin implementation with test coverage.

• #1535868: Convert all blocks into plugins introduces a BlockPluginUI for placing and configuring blocks.
• The goal is to eventually replace much of this interface, but if this does not happen before feature freeze, the existing UI will need test coverage.
• There are no explicit unit-style tests for the class's methods.
• Not all of the UI's functionality has test coverage.
• There is no explicit test coverage for new features of the UI.

Proposed resolution

• Determine which UI functionality is already tested in the Block module's tests.
• Add web tests for those UI features that are not already covered.
• Add explicit unit-like tests for BlockPluginUI methods.
• Use the test implementation added in #1874830: Add a test block plugin implementation with test coverage.

Remaining tasks

Related issues

• #1874830: Add a test block plugin implementation with test coverage
• #1874598: Add BlockTestBase
• #1875340: Review PluginUI architecture
• #1875942: Improve PluginUI documentation