(error "Window system frame should be used")

[Lars Ingebrigtsen]

I'm trying to write some tests for `image-size' (in conjunction with
scaling, which has no tests currently, apparently), but ert just says:

Test image-size condition:
    (error "Window system frame should be used")
   FAILED  1/1  image-size

Ran 1 tests, 0 results as expected, 1 unexpected (2017-04-20 23:29:22+0200)

So I guess in batch mode ... something frame-like has to be set up?  Or
something?  The error message is a bit unclear; I tried looking for
"window system frame" and came up short...

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   bloggy blog: http://lars.ingebrigtsen.no