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.

If the regression comes back, it will only be caught by approvals,
but that's better than nothing.

`REQUIRE`, `CHECK` and many other macros already support expressions
with multiple template parameters without parenthesizing.

isTrue and alwaysFalse were replaced by (void)0, 0 inspired by doctest
alwaysTrue was replaced by warning suppression