Tests (and implementation) for various block constructs need updating

[arporter]

In #226/#227 the tests for the following constructs have been moved into separate files:

Block Do Construct (R826), Non-block Do Construct (R835), Case Construct (R808), Component Part (R438), If Construct (R802), Where Construct (R744).

However, the tests themselves need updating to bring them into line with our development practises. They also need updating to fully cover the various aspects of each construct (in particular, the various conditions associated with them). In many cases this will mean extending the implementation too as it currently does not check for constraints.