Update router-store tests

[MonicaOlejniczak]

The aim of this change is to simplify the router-store tests and reduce overall conflicts with the Router consolidation.

Changes

• Make children prop in MemoryRouter optional, primarily for the tests
• Remove children in tests when they are not asserted
• Rename MemoryRouter to Router in the tests to reduce conflicts later
• Rename spy variable to be more descriptive per test
• Update some test names and overall make things more consistent