Add a test for get_addable_spec

[giordano]

This makes sure we can reliably work around an issue in Pkg.

[giordano]

Uhm, nope:

Setup: Error During Test at /home/runner/work/BinaryBuilderBase.jl/BinaryBuilderBase.jl/test/dependencies.jl:145
  Test threw exception
  Expression: setup_dependencies(prefix, getpkg.(dependencies), platform) isa Vector{String}
  expected package `LibOSXUnwind_jll [a83860b7]` to exist at path `/home/runner/.julia/packages/LibOSXUnwind_jll/avPRF`
  Stacktrace:
    [1] pkgerror(msg::String)
      @ Pkg.Types /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/Types.jl:67
    [2] collect_fixed!(env::Pkg.Types.EnvCache, pkgs::Vector{Pkg.Types.PackageSpec}, names::Dict{UUID, String})
      @ Pkg.Operations /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/Operations.jl:266
    [3] resolve_versions!(env::Pkg.Types.EnvCache, registries::Vector{Pkg.Registry.RegistryInstance}, pkgs::Vector{Pkg.Types.PackageSpec}, julia_version::VersionNumber)
      @ Pkg.Operations /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/Operations.jl:317
    [4] targeted_resolve(env::Pkg.Types.EnvCache, registries::Vector{Pkg.Registry.RegistryInstance}, pkgs::Vector{Pkg.Types.PackageSpec}, preserve::Pkg.Types.PreserveLevel, julia_version::VersionNumber)
      @ Pkg.Operations /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/Operations.jl:1197
    [5] tiered_resolve(env::Pkg.Types.EnvCache, registries::Vector{Pkg.Registry.RegistryInstance}, pkgs::Vector{Pkg.Types.PackageSpec}, julia_version::VersionNumber)
      @ Pkg.Operations /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/Operations.jl:1168
    [6] _resolve(io::Base.DevNull, env::Pkg.Types.EnvCache, registries::Vector{Pkg.Registry.RegistryInstance}, pkgs::Vector{Pkg.Types.PackageSpec}, preserve::Pkg.Types.PreserveLevel, julia_version::VersionNumber)
      @ Pkg.Operations /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/Operations.jl:1203
    [7] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}, new_git::Set{UUID}; preserve::Pkg.Types.PreserveLevel, platform::Platform)
      @ Pkg.Operations /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/Operations.jl:1219
    [8] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; preserve::Pkg.Types.PreserveLevel, platform::Platform, kwargs::Base.Pairs{Symbol, Base.DevNull, Tuple{Symbol}, NamedTuple{(:io,), Tuple{Base.DevNull}}})
      @ Pkg.API /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/API.jl:275
    [9] (::BinaryBuilderBase.var"#88#94"{Bool, Prefix, Vector{Pkg.Types.PackageSpec}, Platform, Vector{String}, Vector{String}})()
      @ BinaryBuilderBase ~/work/BinaryBuilderBase.jl/BinaryBuilderBase.jl/src/Prefix.jl:572
   [10] activate(f::BinaryBuilderBase.var"#88#94"{Bool, Prefix, Vector{Pkg.Types.PackageSpec}, Platform, Vector{String}, Vector{String}}, new_project::String)
      @ Pkg.API /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Pkg/src/API.jl:1678
   [11] setup_dependencies(prefix::Prefix, dependencies::Vector{Pkg.Types.PackageSpec}, platform::Platform; verbose::Bool)
      @ BinaryBuilderBase ~/work/BinaryBuilderBase.jl/BinaryBuilderBase.jl/src/Prefix.jl:565
   [12] setup_dependencies
      @ ~/work/BinaryBuilderBase.jl/BinaryBuilderBase.jl/src/Prefix.jl:539 [inlined]
   [13] macro expansion
      @ /opt/hostedtoolcache/julia/nightly/x64/share/julia/stdlib/v1.8/Test/src/Test.jl:463 [inlined]
   [14] (::var"#37#54")(dir::String)
      @ Main ~/work/BinaryBuilderBase.jl/BinaryBuilderBase.jl/test/dependencies.jl:145