`AssertionError: (Base.JLOptions()).depwarn == 1` when we run the Pkg tests with `Base.runtests(["Pkg"])`

[DilumAluthge]

ERROR: LoadError: AssertionError: (Base.JLOptions()).depwarn == 1
Stacktrace:
 [1] top-level scope
   @ /tmp/jl_kJ4snX/TestArguments/test/runtests.jl:4
 [2] include(fname::String)
   @ Base.MainInclude ./client.jl:451
 [3] top-level scope
   @ none:6
in expression starting at /tmp/jl_kJ4snX/TestArguments/test/runtests.jl:4

The full Buildkite log is here: https://buildkite.com/julialang/julia-master/builds/5345#6729df4a-5129-4510-a957-940bc1ed473c

This is a Buildkite job triggered on the following pull request: JuliaLang/julia#42885

[DilumAluthge]

I think the reason for this is that the Base.runtests function sets the --depwarn=error function: https://github.com/JuliaLang/julia/blob/6c274ed43d9e77606017d7ea4866ba914b4c4f5b/test/testenv.jl#L19

If Julia is started with --depwarn=error, then (Base.JLOptions()).depwarn will be 2.