Remaining dummy tests

[alecandido]

To be removed:

• [ ] keep a test targeting couplers
  • they are not special, but it is relevant to verify they keep being like that
• [ ] remove unrolling
  • as for "multiplexing" (just executing on multiple qubits), unrolling is affecting the overall Platform.execute() result, not the shape of the individual value - this has to be tested separately from test_result_shapes, but it should be established in #964

We should only test as dummy things that are dummy-specific, i.e. not using dummy as a placeholder platform (which could be exchanged for any other working one), but the pieces that are powering dummy itself, in isolation.