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.
This drops matching on the truncated stack trace marker as that only
appears if the stack trace is actually long enough. In some cases the
stack trace is simply too short so Nix prints it out entirely. Then
the tests fail and I'm unhappy.
This was caused by a90d993610 which to the best of my understanding
forces the error message very early in the eval and thus doesn't have
a very long stack trace.
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.
