Simplify the logic for parsing and handling test arguments in the @Test macro

[stmontgomery]

This is a minor refactor of the logic and data structures in the @Test macro implementation responsible for parsing and handling test function argument expressions.

Motivation:

I discovered there was a way to simplify this logic while working on #808, and since this isn't strictly pertinent to that PR I wanted to extract it and land it separately here.

Modifications:

• Remove the need to hardcode the parameter label arguments: in three places.
• Remove the need to iterate a Collection using raw indices.
• Resolve one "TODO" comment.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test