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.

* Removed `IGeneratorTracker::hasGenerator`, as it was never used.
* Removed `IGeneratorTracker::setGenerator`, as actually using it
  after the tracker was used would lead to inconsistent behaviour.
  Instead, the tracker is given the generator it guards during
  construction.