Fixed tests for validation of price_paid.

Note: We need to look at this, as it's a bit of a bodge.
We're currently testing by setting the field.data to the input value.
This doesn't work in all cases, and isn't what Flask WTF does.
We need to pass the form data into the form.
Merge pull request #77 from LandRegistry/fix-price-paid

Fixed tests for validation of price_paid.
Added tests for validating the extent