Subtask of T291062: Make sure all crucial template dialog features have tests.

New outline widgets

• Delete the MWTransclusionOutlineWikitextWidget test. This is all covered by other tests.
• MWTransclusionOutlinePlaceholderWidget needs assertions for the 2 possible messages.
• Add a test for the space bar behavior in MWTransclusionOutlineButtonWidget.
• Make sure MWTransclusionOutlineTemplateWidget.onHeaderClick always works, incl. with empty templates, and when there is only a placeholder.
• MWTransclusionOutlinePartWidget could have tests for all methods. They are all very trivial. Not critical to test, but good to have nevertheless.
• Make sure MWTransclusionOutlineTemplateWidget doesn't create checkboxes for placeholders or deprecated parameters (except they are used).
• Add more edge-case test cases for MWTransclusionOutlineTemplateWidget.findCanonicalPosition.
• Is it worth having a test for the tabIndex behavior in MWTransclusionOutlineParameterSelectWidget?
• Test space bar behavior in MWTransclusionOutlineParameterSelectWidget.
• Cover both code paths when pressing enter in MWTransclusionOutlineParameterSelectWidget. The code path is different when the parameter is checked vs. when it's not.
• Make sure MWTransclusionOutlineContainerWidget.onTransclusionModelChange is covered with all possible edge-cases.
• Make sure MWTransclusionOutlineContainerWidget.addPartWidget ability to place items at specific positions is covered with all possible edge-cases.
• Is it worth covering all (custom) events MWTransclusionOutlineContainerWidget can emit?