Consider reinstating tests for invalid channels

[ns-rse]

#24 removed tests that checked that the available channels were reported when an invalid channel was specified for a
given tile-type.

Whilst the functionality may still be retained i.e. users get useful information if the wrong channel is specified it
would perhaps be sensible to reinstate the tests that we had so that we are confident this is the case and when new
file types are supported we can check that they too have the available channels reported.

[ahobbs7]

I removed those tests as it is now impossible to input a channel which is not in the available channels as the available channels are given as a dropdown, so the user can only select a channel from the ones which are available.

[ns-rse]

Ah ok, that is a good reason to remove them and means this issue can be closed out quickly.

Apologies for not picking that up from the commit history/changes.