Integrate Generators into reporters and CLI

Unlike Sections, Generators are currently invisible to reporters, and
also to the CLI. This means that in test case like this

```cpp
TEST_CASE() {
SECTION("a") { ... }
auto i = GENERATE(1, 2, 3);
SECTION("b") { ... }
}
```

the user can ask to run specifically "a" section, or the "b" section 3 times,
but they cannot ask to run the test case with `i == 2` (whether they
want to run a specific section or not).

This then makes errors in generator-based tests harder to debug than they
should be (debugging the test case for 17th element out of 23 is hard to
set up).

This is an important step towards death tests, which require ability to
run specific test in a different process.