We test a lot of different ways. We test characters alone, self buffed and raid buffed. We do very controlled tests and we do tests where we just tell a skilled player to go nuts and we do actual raids. We get a lot of information from outside Blizzard as well, but you asked about how we test.
It's difficult to know exactly when a tank is going to end using their cooldowns. Sometimes it's probably fine to spam them and other times they need to be saved for a very specific situation. Which abilities you use is also determined by what classes are present in the rest of your group. That's just another reason to gather a large body of data.
We think the paladin and warrior are pretty close now. I do have some concerns about eventual scaling problems now that block is so good. Critical Block may end up letting warriors bypass paladins eventually, but that will be a few raid tiers from now, so we'll have plenty of time to evaluate the situation and make adjustments if necessary.
You're still going to have a hard time convincing me that 2% mitigation difference will determine which class everyone uses for their MT. Consider:
1) You're going to have a hard time even knowing what that number is. Unless a few really eloquent theorycrafters manage to convince the entire raiding community, most of the time you're going to have to rely on actual raid data to make decisions, and that kind of variance is going to be really hard to detect.
2) Every boss is different. Mitigation deltas may be miniscule on a slow-hitting boss but noticeable on a fast-hitting boss for instance.
3) A difference likes 2% assumes both tanks have the best gear possible for their slot. Gear can make a very big difference in mitigation, yet you tend to see guilds stick with their MT for a long time. You don't suddenly drop your guy when someone with a better shield and 10% more mitigation comes along. Why? Because your guy's loyalty, dependability, knowledge or sense of humor is more important than that 10% difference.
4) Don't forget player skill has a huge role in here too. I've done Archimonde with an MT that couldn't stance dance to save his life. But despite that er... handicap, the group is on M'uru or something now. Yeah I know, everyone has antecdotes. The moral of my story is that if a tank who can't use his cooldowns can make it to Sunwell, surely the tank with 2% less mit can. If min-maxing tumped all, they would have dumped him.
5) Paladins and druid tanks are already tanking a lot of content in BC, and that's in spite of the current design (i.e. the BC version, not the LK one) that warriors are the best single boss tanks. And, in spite of things like crushing blows. And, in spite of the difficulties druids and paladins had getting gear. If they can tank already when they're supposed to be OTs, then they should probably be great when we are actively trying to make them MTs.