This drops the stack trace truncated marker from all the fixtures as
those aren't reliable. What we care about is the exact error message
and not the stack traces.
In 53cd6f263c the oldestSupportedRelease
version was bumped to 26.05 which caused the meta/problems tests to
fail because the `config.allowBrokenPredicate` is now deprecated and
the code is warning about it.
This was an oversight in https://github.com/NixOS/nixpkgs/pull/478539
that became apparent in https://github.com/NixOS/nixpkgs/pull/494416:
If there's a failing problem in Nixpkgs packages, CI will call
handleEvalIssue on it, but the problem error was missing a `.reason`.
Though even if it did have a reason, we need to do more to make sure
we don't break any code that uses it, so the new code uses the problem
kind as the reason, which happens to match with the reason for all
expected problem kinds.
