[release-1.12] Add test for workspace sources pointing to parent package

[IanButterworth]

Testing the test in #4568 without the fix

Confirmed:

workspace sources pointing to parent package: Error During Test at /Users/runner/work/Pkg.jl/Pkg.jl/test/workspaces.jl:191
  Got exception outside of a @test
  AssertionError: normpath(entry.path) == normpath(path)
  Stacktrace:
    [1] write_env(env::Pkg.Types.EnvCache; update_undo::Bool, skip_writing_project::Bool)
      @ Pkg.Types ~/work/Pkg.jl/Pkg.jl/src/Types.jl:1292
    [2] write_env
      @ ~/work/Pkg.jl/Pkg.jl/src/Types.jl:1283 [inlined]
    [3] up(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}, level::Pkg.Types.UpgradeLevel; skip_writing_project::Bool, preserve::Nothing)
      @ Pkg.Operations ~/work/Pkg.jl/Pkg.jl/src/Operations.jl:1986
    [4] up
      @ ~/work/Pkg.jl/Pkg.jl/src/Operations.jl:1957 [inlined]