Support for data test gutter icons

[alfonsoristorato]

Adds support to enable gutter icons when data tests are used.
However:

• Because we need the test names in order to be able to run them singularly, we never had gutter icons for data tests, as their names are defined at runtime, at which point it is too late to run only the selected one. This PR introduces the gutter icon, but it runs all tests within the spec.

Future developments:
This PR introduces the foundation for the gutter icon, but in future (assuming possible) we could:

• somehow only run all the data Tests by excluding all other tests for which we have defined names (EXCLUDE clause)
• somehow only run selected data Tests by running tests from index x to index y, where x is the first data test parent from where the user clicks the icon, and y is the index of the next defined test (if any) or last test. These two ideas are just ideas for now, and are to be taken as such. We might also decide to leave the gutter icons as they are.