Add choice "ignore" to update golden value "yes/no"

[andreasabel]

Sometimes a golden value isn't an exact match to the test output, but I ran under special circumstances (e.g. different name of the Agda executable). In this case, I do not want to update the golden value, but I nevertheless want to pass the test successfully. Thus, the two choices

• y: update golden value, test succeeds
• n: do not update golden value, test fails

should be complemented by

• i ("ignore"): do not update golden value, test succeeds