A RuleRunner subclass to set ICs in Python-related tests.

[benjyw]

Subsequent PRs will apply this in relevant tests, so we can remove
one of the hacks introduced in #18627.

I have applied this to several tests and verified that it works.

[Eric-Arellano]

Maybe make this a private file? We probably don't want a new API that plugin authors come to depend on and now we have to deprecate.

[benjyw]

Maybe make this a private file? We probably don't want a new API that plugin authors come to depend on and now we have to deprecate.

Well, but this is a useful class for plugin authors. Why would we have to deprecate it?

[Eric-Arellano]

Why would we have to deprecate it?

Yeah I guess the plugin API still lets us delete things without deprecation

[kaos]

Cool. Done exactly as what I had in mind 😎

[kaos]

Why would we have to deprecate it?

Yeah I guess the plugin API still lets us delete things without deprecation

Eric, I think you got the question backwards. At least I think it was "why would we want to delete it?"

[benjyw]

Why would we have to deprecate it?

Yeah I guess the plugin API still lets us delete things without deprecation

I mean why would we delete it? It's pretty useful.

[benjyw]

To clarify, this isn't a temporary thing. At least, I don't expect it to be.

[Eric-Arellano]

Ah, I thought this was meant to be temporary while you fix individual tests. Sg.

[benjyw]

Ah, no, this is the fix. Seemed better than repeating a fiddly option set up in dozens of places.