Give a warning and a failure instead of an UnDefinedVarError if GTK is not loaded

[oxinabox]

I was very confused that i kept getting:

┌ Warning: Should we make this the new reference image?
└ @ VisualRegressionTests ~/.julia/packages/VisualRegressionTests/q0Rxt/src/imgcomp.jl:39
LayeredLayouts.jl: Error During Test at /Users/oxinabox/.julia/packages/VisualRegressionTests/q0Rxt/src/macros.jl:18
  Test threw exception
  Expression: test_images(testFilename, ref("1"), popup = true, tol = 0.02) |> success
  UndefVarError: replace_refimg_dialog not defined
  Stacktrace:
   [1] test_images(::String, ::String; popup::Bool, newfn::String, kw::Base.Iterators.Pairs{Symbol,Float64,Tuple{Symbol},NamedTuple{(:tol,),Tuple{Float64}}}) at /Users/oxinabox/.julia/packages/VisualRegressionTests/q0Rxt/src/imgcomp.jl:40

when testing things.
I assumed there was a bug in the package.
But then i looked in the source and saw that Requires.jl is being used to include the file with that function defined in it, only if GTK was loaded.

We should be able to use isdefined to check if that function is defined.
and if it isn't then rather than having an undefined error,
say something like "Please load GTK to regenerate images"

[FelixBenning]

I can not get it to work at all, even after I add Gtk to the package requirements I still get this error.

[kellertuer]

On my new machine I am running into the same problem and have not yet found a way to make it work.

[oxinabox]

You need to do using Gtk as well as using VisualRegressionTests

[kellertuer]

Ah, I forgot the using Gtk – I don't know why it ever worked without that; now it worked. Thanks for providing this issue that I found what the error message means :)