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

Originally the tests were from #1912, but as it turned out, the issue
was somewhere else. Still, the inputs provided were interesting, so
they are now part of our test suite.

It did not clear out all of its internal state when switching from
one pattern to another, so when it should've escaped `,`, it took
its position from its position in the original user-provided string,
rather than its position in the current pattern.

Fixes #1905

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

Add both `[.]` and `[!hide]` tags when registering a hidden test case, as per documentation.

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

Fixes #1809

b77cec05c0 fixed this problem for tagging tests, so that a test
case tagged with `[.foo]` would be parsed as tagged with `[.][foo]`.
This does the same for the test spec parsing.

Fixes #1798