Add Unit Tests for Immutable Validate Class

**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 
    1. ImmutableValidated vs. Tval

**Value Proposition**

More confidence in the code & gives us more scope to refactor in future.