Previously the mapping was eager on construction and calls to
`next()`. This was fine when generators were always exhausted fully,
but with the new generator filtering, we might not want to call
the map function eagerly; rather we want to call it only once,
after the generator is moved to the target element.

Not being able to filter generators to specific element has been regularly
causing problems. It was possible to use a dynamic section to run tests
for specific element in a generator, at least if the element had a nice
string representation, but the test case would still run once per element
in the generator.

With this change, it is possible to have the generator return only one
specific element, and do so based on the index, rather than the string
representation of the element. This enables simple debugging of tests
that fail for specific generator element.

This will be useful later to implement warning on infinitely
running `GENERATE` expressions.

* Add validation for --benchmark-samples to prevent crash with zero samples
* Add automated test for benchmark samples validation
* Update baselines for benchmark-samples validation
* Add missing test for benchmark samples validation

---------

Co-authored-by: Chan Aung <chan@Thinkpad.localdomain>

Specifically, this commit makes the `-c`/`--section` parameter
strictly ordered and hierarchical, unlike how it behaved before,
which was a huge mess -- see #3038 for details.

Closes #3038