I'm not sure the criteria for determining an 'average' pilot but if becoming a test pilot is a big incentive for trying to be the best you can wouldn't this drag the average pilot quality down?
You'll want the good ones for that first task.
The reality is probably more like for developers, some of the best developers in terms of achievements are not people you'd want to work with on a daily basis. In the same way that an F1 driver wouldn't necessarily be the world's best bus driver.
The principle for pilots is similar.
As to skills assessments, you're right, it's difficult. But you can use some general proxies. Certification pilots should be drawn from a standard pool, not a test-pilot pool. Some sense of performance on some standardised battery of qualification flights or simulator check-flights might be used, with scoring based on success or performance in identifying and addressing conditions encountered.
The fact that a precise quantity cannot be confidently stated doesn't mean that no quantity or range can be given.
And for the sake of the certification pilots, I'm assuming check-out flights would either be in simulators or with a more qualified check pilot assisting.
An ideal test pilots should excel in stick and rudder skills and have the ability to work around mechanical / system problems.
For a line pilot attributes like organisational skills and planning, interpersonal skills and meteorology skills are much more valuable.