[Testing] More tests / refactor fill_identities

[nmheim]

The fill_identities function is a transpilation pass that adds an identity block to every "empty" slot in a circuit. Currently it is only used for the drawing system.

It may need a refactor. Currently its a bit complicated/ugly. The following examples can be turned into tests:

b = chain(CNOT(0,1), CNOT(1,2))
b = chain(kron(CNOT(0,1)), kron(CNOT(1,2)))
b = kron(CNOT(0,1))
b = chain(SWAP(0,1), SWAP(0,3))

[mlahariya]

@jpmoutinho Check if relevant and could be added to #613.