Skip some stock GC-specific tests when using MMTk

[udesou]

As noted in #57103, there are a few tests that are specific to the stock GC and will fail for MMTk. For now, this PR simply skips those tests when not using the stock GC until we agree on how to approach the incompatibilities described in the issue.

[d-netto]

Any chance we could avoid the duplication of const USING_STOCK_GC...?

[gbaraldi]

That can probably be a global defined in Base or Sys

[udesou]

gcutils.jl maybe? Let me try that out. That didn't work, I got an undefined var error. I moved it to Base instead.

[d-netto]

For the tests in test/gc.jl, I also wonder whether it would be possible to guard these lines with a @static if ....

@testset "GC page metrics" begin
    run_nonzero_page_utilization_test()
    run_pg_size_test()
end
@testset "issue-54275" begin
    issue_54275_test()
end
@testset "Base.GC docstrings" begin
    @test isempty(Docs.undocumented_names(GC))
end
@testset "Full GC reasons" begin
    full_sweep_reasons_test()
end

As opposed to the individual function bodies.

[NHDaly]

Maybe we should also expose GC.gc_active_impl() which returns the string, and then have a separate function GC.is_stock_gc() that does this occursin?

[udesou]

I can add GC.gc_active_impl(). However, I got an undefined var error when I tried adding something with occursin to the GC module in gcutils.jl. I assume due to that function only being imported later on, and I wasn't sure if I should change that order...

[d-netto]

Please fix the grammar NIT.

Other than that, LGTM.