I think it does make them more credible. Our early astronauts came from the test pilot programs, mostly, where careful observation of things (expected or not) out of the ordinary was a critical life skill, not to mention part of their job.
If Bob says he saw a UFO, I am likely to believe that he might have been drunk, or mistaken, or tricked by the light. If a test pilot (who is more familiar with light effects and weather balloons, etc) or astronaut tells me they saw something out of the ordinary, I lend that more credence.
Of course it's possible that they're mistaken in their testimony, but I think it's more likely for the astronaut to have noticed and correctly reported something than for the average Joe.
Whatever your experience, there always is something that is so rare that you won't have seen it. Bob's will have that with an event of which the test pilot thinks "I have seen that countless times", that test pilot will have it with an event that is way rarer, but that still may have some quite ordinary explanation. Program is, we don't have an super test pilot who can say "I have seen that countless times".
And yes that will be something out of the ordinary. But the test pilot claims as to what it is, I wouldn't trust more than Bob's claims as to the nature of the things he finds extraordinary.
The real thing you should ask is, did "Bob" believe in UFOs before he/she became a pilot, or involved in the military? The military doesn't give people any more information than they need, there's no reason to think that a highly trained pilot has any more information than they need to fly a very fast deathplane :)
If Bob says he saw a UFO, I am likely to believe that he might have been drunk, or mistaken, or tricked by the light. If a test pilot (who is more familiar with light effects and weather balloons, etc) or astronaut tells me they saw something out of the ordinary, I lend that more credence.
Of course it's possible that they're mistaken in their testimony, but I think it's more likely for the astronaut to have noticed and correctly reported something than for the average Joe.