test: Add some tests for dependency suggestions

[claremacrae]

Description

Motivation and Context

• Replace an Obsidian-specific string.contains() with string.includes() which is available when run via tests.
• Add some initial tests to capture the behaviour of auto-suggest facilities added in feat: Add Auto Suggest for Task dependencies #2771.
• Add a fake implementation of prepareSimpleSearch() - with its own tests - so that I could test suggestions made for the dependsOn field.

How has this been tested?

• Exploratory testing to confirm the actual behaviours of the tests I've added
• Automated tests

Types of changes

Internal changes:

• Refactor (prefix: refactor - non-breaking change which only improves the design or structure of existing code, and making no changes to its external behaviour)
• Tests (prefix: test - additions and improvements to unit tests and the smoke tests)

Checklist

• My code follows the code style of this project and passes yarn run lint.
• My change has adequate Unit Test coverage.

Terms

• My contribution follows this project's contributing guide
• I agree to follow this project's Code of Conduct

[claremacrae]

Hi Ilyas, FYI.... I'll merge this later anyway, as I need to share it with the author of #2771 - but low priority suggestion - if there is anything really unclear I would appreciate any quick feedback you could offer...