Clean up pass on geometry state tests (RobotLocomotion#11546)

* Clean up pass on geometry state tests

This addresses a number of style and cosmetic issues encountered while
integrating rendering into geometry state. This isolates the legacy issues
so that new code can be considered without this noise in the signal.

This doesn't change the *functionality* of the tests. It only changes
spellings and the like:

1. Use of aliases to make code more compact.
2. Added const to tons of local variables.
3. Refactored the GeometryStateTest core into a new calss:
   GeometryStateTestBase (this will facilitate new tests in upcoming PRs).
   - Change the interface to SetUpSingleSourceTree() to make call sites
     more readable.
   - Remove to methods that are no longer used:
     - ExpectSourceDoesNotHaveFrames()
     - AssertSingleTreeCleared()
   - Rename vectors of poses: e.g., X_WF_ --> X_WFs_.
4. Make more fluent use of pose data (will lend itself to translation to
   RigidTransform).
5. Clarify, clean up comments.
6. Move ProximityRoleOnMesh to bottom.