Refactor for Pipeline::Expression::Lexer spec

What does this MR do?

I came across this Lexer spec while working on something else and thought it could use some restructuring. I added subjects and more context blocks to change out the expression being used in any given context. The context blocks also made it easy to break up some of the specs into multiple with fewer expectations in each.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers