use ScopedValues for TestSets

[simonbyrne]

Another attempt at #51012

[simonbyrne]

@vchuravy @Keno I'm getting some weird failures on this branch originating from this line:

julia/base/compiler/ssair/passes.jl

Line 1162 in c379db7

@assert current_mapping == val

The simplest reproducer I have is:

using Test
@testset begin
    @testset for r in Int[1]
    end
end

[simonbyrne]

Blocked by #53521

[nickrobinson251]

Would love to see this merged!

@simonbyrne do you have any time to work on this? I'd be happy to rebase and help get it over the line. I think rebasing, and then deciding what to do with the commented-out scope tests might be all that's needed here?

[giordano]

I initially thought I managed to badly botch the rebase, but I believe this is now failing to build because after the rebase #55452 was merged in and that PR removed using .ScopedValues inside Base, so imports need to change a little bit. I'll leave this exercise to someone else 😁

[simonbyrne]

Sorry, it still needs some work. The handling of errors is wasn't quite correct, and the resetting of rng state is kind of confusing (arguably should be handled differently)

[simonbyrne]

Currently blocked by #56062