the book says the following for the spell: The target unit must pass a Strength test every time it moves, shoots, casts spells, ... how to take this test, when there are models with different strength in the unit?
I would be inclined to take multiple tests, 1 for each different value. The likelihood is that such a unit will have standard rank and file and one or more character. All pass then they can move etc, if only the character passes then they can shoot, move etc but not the unit.
This spell is fairly unique. Most "take a str test" or whatever is based on each model in the unit, or everything under a template. This spell just says "the unit" which under the brb you use the highest to take the test (just like you get to use the highest leadership in the unit). You would not need to make multiple tests. It makes the spell less attractive to cast at units with characters in it I suppose. Edit: Page 10 of the BRB, last paragraph of characteristics tests. Nothing in the character section mentions anything further. The only other entry regarding tests is under warmachines.