Adding "large" to PG_TEST_EXTRA

Hi,

I'm working on rebasing [1], my patch to make relation extension scale
better.

As part of that I'd like to add tests for relation extension. To be able to
test the bulk write strategy path, we need to have a few backends concurrently
load > 16MB files.

It seems pretty clear that doing that on all buildfarm machines wouldn't be
nice / welcome. And it also seems likely that this won't be the last case
where that'd be useful.

So I'd like to add a 'large' class to PG_TEST_EXTRA, that we can use in tests
that we only want to execute on machines with sufficient resources.

Makes sense?

Greetings,

Andres Freund

[1] https://www.postgresql.org/message-id/20221029025420.eplyow6k7tgu6he3%40awork3.anarazel.de