Streamline reduce_tests.sh a bit

[kevingranade]

Summary

None

Purpose of change

When I tried to use reduce_tests.sh on #81077 I found it to be quite fragile and require a lot of babysitting for false positives (a positive in this case is the test fails in an expected way).

Also I found that the multidelta wrapper script was re-running the test suite redundantly under the assumption that it was doing test case reduction across multiple files and using a tool to semantically group c/c++ expressions, neither of which we were doing.

Describe the solution

Adds a stub showing how to test for a target output string instead of test success. This is the recommended way to use delta to avoid false negatives, I think it can handle false negatives from "this makes the build fail" or "this makes the whole thing crash", but false positives seem to make it wander off into un-fruitful directions.

Swaps from multidelta that makes things slower without giving any new features to singledelta.

Describe alternatives you've considered

Ideally this would be goverened by an argument or something instead of requiring manual editing, but I really do not have the brain juice for that right now.
Since this isn't running through the "flatten" process any more, I don't think adding and removing curly brackets around the test cases is necessary anymore, but it's also not breaking anything so whatever.
Possibly the "match against a string" workflow should be the default, but that requires parameter handling so bleh.

Testing

Ran it a bunch of times against #81077 and it was pretty reliable once I had it searching for the error string.

[github-actions]

Auto-requesting reviews from non-collaborators: @jbytheway

[ehughsbaird]

This is a good add! I've not had much of a problem with misc failures polluting the tool, but it has happened once or twice.

It's also good to see I'm not the only one who uses this :)

[jbytheway]

FWIW, the reason I used multidelta was, as ehughsbaird says, that the other script has different names in different distros.

But I'm also glad to see this script is still seeing some use.

[ehughsbaird]

I think this is portable.

[kevingranade]

Thanks! I've just been trying to keep focused on hordes so neglecting this a bit, but I'll go ahead and merge this in.