Coverage: Clean up ModBreaks treatment

This fixes a few issues with out treatment of ticks (and breakpoints):

• Eliminate an unnecesary (and arguably wrong) filename check
• Fix a thunk leak due to the tickBoxCount field
• Refactor and consolidate the logic for adding box ticks to the environment
• Avoid producing ModBreaks if we didn't produce any breakpoints

The latter point could be considered to be a bug-fix as emptyModBreaks (which was previously produced) contains a bottom that is relatively easy to hit. ghcide hits this fairly regularly, for instance.

Fixes #18229 (closed).