UnitTest_Engine: Modify methods with void return type not supported in check test

#### Description:

As title states, need to make a special case for checking of methods with void return type.

Linked to https://github.com/BHoM/BHoM_UI/issues/430

#### Steps to reproduce:

try checking a test with void return

#### Expected behaviour:

For modify method with void return we need to:
Clone the inputs before running
Take the first argument of the inputs after running and put that as the output from the run

#### Test file(s):