[test] #213: modify patched_get_source()

* modify values to facilitate test writing
* rename `amount` variable to `balance`

* create test_constants file to prevent circular dependencies
    * modify relevant files : import mock_ud_value from patched.test_constants instead of patched.money

* patched_get_sources() now has a `counter` attribute, and can return different values if called twice
    * can mock the result of an intermediary transaction.

* mod test_get_list_input_for_transaction()
  * to reset the patched_get_sources counter.
  * to make values match new patch behavior
  * to comment what is tested