Tag limitation, and effects/rules that affect all Tests
As of now, the only effect/rule that affects all D20 Tests is the new rule on Inspiration. The fact such a rule exists does imply other rules or effects could be released that target D20 Tests as a whole. As a matter of fact, such effects already exist in 5e, without using this umbrella term.
Tagging questions regarding such effects with the current tags attack-roll, saving-throw and skills (thanks to Thomas Markov for pointing that last one out, since there's no tag for ability checks specifically as of now) would require 3 tags. Knowing that there's only 5 slots per question, that is a sizeable amount that, in my opinion, would be a good reason to use a single tag for questions that target Tests, and not one of those specific kinds of rolls.
Because there's only one rule that affects all Tests, this tag might have little use in the short term, but it would definitely benefit more questions later on, when more rules are released regarding those Tests.