DAL: test amplification with --ignore-pkhs

What

Add a new test for the amplification. The current test is simpler than the one banning peers. It also publishes more slots (ie 3) and checks that they are correctly attested.

It also includes factoring out a piece of code that was repeated often.

Why

This checks that --ignore-pkhs works properly.

This was used to test !18329 (merged) (additionally adding manual instrumentation).