Add tests for toqito/state_props/is_separable.py: 77->84, 114->128, 136-188

[purva-thakre]

https://github.com/vprusso/toqito/blob/70ecd264508824d0991fb954d41020a7b9a915a2/toqito/state_props/is_separable.py#L77-L81

Above ValueError can be removed.

https://github.com/vprusso/toqito/blob/70ecd264508824d0991fb954d41020a7b9a915a2/toqito/state_props/is_separable.py#L114-L125

https://github.com/vprusso/toqito/blob/70ecd264508824d0991fb954d41020a7b9a915a2/toqito/state_props/is_separable.py#L135-L188

[Yash-10]

#612 added more conditions in is_separable, but the test coverage needs to be improved. In particular, these lines are currently untested:

93->100, 127->139, 148, 159-182, 189-253, 273->295, 291, 297-306, 310

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L93-L100

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L127-L139

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L148

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L159-L182

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L189-L253

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L273-L295

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L291

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L297-L306

https://github.com/vprusso/toqito/blob/3d20a16d506f63f934555efc5cdc6865ddd45ffe/toqito/state_props/is_separable.py#L310

[vprusso]

Great additions to the issue, @Yash-10 . Much obliged for adding those!