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.

As with other Bazel env vars, it overrides the corresponding CLI
parameter if both are set.

Closes #3021
Closes #3024

This tells Catch2 to create an empty file at specified path before
the tests start, and delete it after the tests finish. This allows
callers to catch cases where the test binary silently exits before
finishing (e.g. via call to `exit(0)` inside the code under test),
by looking whether the file still exists.

Closes #3020

Previously it could be just plain reporter name, e.g. `xml`, but
it could not specify other reporter options. This change is not
particularly useful for the built-in reporters, as it mostly comes
in handy for combining specific custom reporter with custom arguments,
and the built-in reporters do not have those.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1