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 avoids the problem where writes to stderr/stdout stop being
line-buffered when stderr/stdout is redirected to a file, which led
to different order of outputs between Linux and Windows in our tests.
Martin Hořeňovský