[Testing] More tests for `fill_identities`

[nmheim]

Add tests for fill_identities. namely turn the following examples 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))

This could also include a refactor of how fill_identities works. Currently its a bit complicated/ugly.