Add Unit Tests for Immutable Validate Class

[ColmBhandal]

Requirement

Add some unit tests including but not limited to:

1. Test that validated function called on construction
2. Test that getting the value doesn't change the value
3. Test equality
   1. ImmutableValidated vs. ImmutableValidated
   2. ImmutableValidated vs. Tval

Value Proposition

More confidence in the code & gives us more scope to refactor in future.