Add unit test for the basic functionality

Problem/Motivation

As we have plans to continue adding new features and keep things working properly on all Drupal versions supported adding the unit test is a way to achieve this goal.

Proposed resolution

Create a Webbrowser test to access some examples of webforms or forms configured to use the calculation field elements.
Create the following scenarios to be validated.

• Create a unit test to validate the basic math expressions like, +, -, / and *. Something like: :field1 + :field2 * 10. Try to explore alternative scenarios to validate
• Validate these expressions on the webforms and form APIs.
• Also validate the expression with markup elements.
• Create a unit test to validate the admin area, checking the field settings are been validated properly.
• Create a merge request to develop branch

Remaining tasks

Maybe we gonna add more scenarios to be covered on the unit tests.