Improve test suite 2

[alexevanczuk]

Summary

This is the first of two PRs to clean up the test suite.

This PR does the following:

1. It takes common test setup that is an implementation detail (and thus distracts from the behavior of the test) into a common helper
2. It cleans up some let blocks
3. It removes an unnecessary, redundant context
4. It adds some test cases for edge cases that are not covered whose behavior might change when I open a PR for danger-packwerk does not add an inline comment on deprecated_references.yml under some circumstances #2

The next PR will:

1. Improve it block descriptions to be more clear about what the expected outcome is
2. It will create a custom matcher have_inline_markdown that encapsulates some common test details, such as:

• It ensures there are no other types of danger messages besides markdown
• It identifies what line number the comment is on so its simpler to visualize the danger message

Commits

• clean up common test setup
• clean up let blocks
• remove unnecessary context
• Add some test cases

[technicalpickles]

This will make a follow up PR much easier to read, thanks!

Left two suggestions on spelling of the filename, but those were also already there.