runtime consolidation: refactoring

[davidjgoss]

This PR makes some non-breaking refactors around the runtime area, namely:

• Inclusion of a workerId with a message is now the responsibility of the runtime-agnostic test case runner, so we don't have messages being created across many parts of the code
• Use of a single coordinates block rather than passing around all three arrays of paths everywhere

Small first step to #2357.

[coveralls]

coverage: 98.395% (-0.002%) from 98.397%
when pulling e41ba5a on feat/runtime-consolidation-part-1
into 2c147a8 on main.