You start from the assumption that it's in anybody's interest that these machines do actually work. It isn't.

The real interest is for well-connected companies to sell equipment; for the approving bureaucrat to get his kickback while not being seen as abusing his position; and for the low-level unskilled TSA employee to keep quiet and keep cashing his salary.

The suggestion you give would mean the above-mentioned bureaucrat should spend additional money (which would require a lengthy fight with his superiors in order to get additional budget, possibly even triggering a review of previous expenditure) in order to push his underlings to point out flaws in the equipment he bought, so that... he can be made a fool of ?

So you see why it isn't going to happen any time soon.

"Was there any financial bonus for the screeners to find the fake guns and bombs? For example, would a screener get $100 if they found one of these planted items?"

In about 5 seconds after such a bonus program was initiated, screeners would be offering kickbacks to the testers in return for being given advance notice of the test.

Surely the TSA screeners do not know who the testers are, or what they look like. That would defeat the purpose of the test.

Then the testers should be given a financial incentive to get the items through.

Then long wait times would be increased dramatically by screeners trying to be as thorough as possible.

I doubt it, minimum wage jobs don't usually have bonuses. Have you ever worked in one? Walmart has no bonuses either.