Allow custom testset for all package tests

[evetion]

instead of requiring a @testmodule, adding it to each testitem and then nesting the tests with a custom testset for each individual testitem

@testmodule CustomTestSet begin
    include("customtestset.jl")
end

@testitem "Test" setup = [CustomTestSet] begin
    @testset CustomTestSet.CustomTestSet "Custom" begin
        @test true
        @test false
    end
end

it's much nicer to do:

@run_package_tests testset = CustomTestSet

Related to #99