check_equal for DoesNotExist

[mzgubic]

We have check_equal for Zero() but not for DoesNotExist(), which currently breaks

julia> check_equal(2.0, DoesNotExist())
ERROR: MethodError: no method matching length(::DoesNotExist)
...

julia> check_equal(Zero(), DoesNotExist())
ERROR: StackOverflowError:
...

We should probably have DoesNotExist() be equal to itself, and nothing else

check_equal(::DoesNotExist, x; kwargs...) = @test false
check_equal(x, ::DoesNotExist; kwargs...) = @test false
check_equal(x::DoesNotExist, y::DoesNotExist; kwargs...) = @test true

[nickrobinson251]

@oxinabox and @willtebbutt didn't like check_equal for DoesNotExist in #57 (comment), but maybe this proposes different answers?