The problem was that under specific circumstances, namely that none
of their children progressed, `GeneratorTracker` will not progress.
This was changed recently, to allow for code like this, where a
`SECTION` follows a `GENERATE` at the same level:

```cpp
SECTION("A") {}
auto a = GENERATE(1, 2);
SECTION("B") {}
```

However, this interacted badly with `SECTION` filters (`-c foo`),
as they could deactivate all `SECTION`s below a generator, and thus
stop it from progressing forever. This commit makes GeneratorTracker
check whether there are any filters active, and if they are, it checks
whether its section-children can ever run.

Fixes #2025

This means that code such as

```cpp
TEST_CASE() {
    SECTION("first") { SUCCEED(); }
    auto _ = GENERATE(1, 2);
    SECTION("second") { SUCCEED(); }
}
```

will run and report 3 assertions, 1 from section "first" and 2
from section "second". This also applies for greater and potentially
more confusing nesting, but fundamentally it is up to the user to
avoid overly complex and confusing nestings, just as with `SECTION`s.

The old behaviour of `GENERATE` as first thing in a `TEST_CASE`,
`GENERATE` not followed by a `SECTION`, etc etc should be unchanged.

Closes #1938

The underpowered and oversubscribed CI servers are hell.

Closes #1954

* Successive executions of the same `GENERATE` macro (e.g. because
of a for loop) no longer lead to multiple nested generators.
* The same line can now contain multiple `GENERATE` macros without
issues.

Fixes #1913

This brings our output inline with GTest's. We do not handle skipped
tests properly, but that should be currently less important than
having the attribute exist with proper value for non-skipped tests.

Thanks @joda-01.

Closes #1899