Promote test i686-linux-gnu[net] and gcext to must-pass

[IanButterworth]

• Promotes test i686-linux-gnunet and test i686-linux-gnu
• Promotes gcext

Closes #325

[IanButterworth]

Same failure 3 times in a row on test i686-linux-gnu

  | 2024-06-21 13:47:24 EDT | misc                                             (10) \|         failed at 2024-06-21T17:47:24.853
  | 2024-06-21 13:47:24 EDT | Test Failed at /cache/build/tester-amdci5-10/julialang/julia-buildkite/julia-01af3e921f/share/julia/test/misc.jl:590
  | 2024-06-21 13:47:25 EDT | Expression: abs(summarysize(z) - 640000) / 640000 <= 0.01
  | 2024-06-21 13:47:25 EDT | Evaluated: 0.18746875 <= 0.01
  | 2024-06-21 13:47:25 EDT

[IanButterworth]

Blocked by JuliaLang/julia#54606 (comment)

[IanButterworth]

Seems like this intermittent issue should also be a blocker JuliaLang/julia#54450

[vtjnash]

We can mark that test_skip or @test skip=true as that test has never worked correctly on i686 (but we missed out on that because it wasn't a must-pass test)

[IanButterworth]

Hit another failure now

math                                              (7) |         failed at 2024-06-23T13:58:26.792
Test Failed at /cache/build/tester-amdci5-8/julialang/julia-buildkite/julia-2d4ef8a238/share/julia/test/math.jl:1607
  Expression: Core.Compiler.is_foldable(effects)
     Context: effects = (+c,+e,+n,!t,+s,+m,+u,+o)
              T = Float64

[IanButterworth]

JuliaLang/julia#54910