What does a test (difficult or challenging) advance exactly? Does it mean advancement for both skill and its respective stat, or does it only advance either skill (if you have it) or stat (if you don't have a corresponding stat and are doing a Beginner's Luck roll)?
Tests are logged per ability, which could be a skill, a stat, or another attribute like Steel or Circles. When the appropriate number of each kind of test is reached, that associated ability advances. Skill tests only advance the exponent of the associated Skill, and so on.
Beginner's Luck is logged in a special way which depends on whether the test is routine—which counts as a test toward learning the Skill—or difficult or challenging—which counts towards the Stat. You need a number of (routine) Beginner's Luck tests equal to the skill root's aptitude to learn the skill, at which point the skill opens at half the root Stat (rounded down).
Let's say you're learning Persuasion, which is rooted in Will. You have a B4 Will. Your Will Aptitude is 10 - Will which is 6. You need to make six routine Beginner's Luck Persuasion tests rolling your Will, upon which you will open Persuasion at B2. Tests that count as difficult or challenging will instead count as difficult or challenging tests toward Will—they don't help you learn Persuasion at all (or at best indirectly, as it could advance Will which would lower the Aptitude).