Fix minor testing path bug

[IanButterworth]

This fixes

Errors encountered while save File{DataFormat{:PNG}, String}("/home/ian/.julia/packages/ObjectDetector/sX9Lc/test/results/dog-cycle-car_nonsquare/v2_tiny_416_COCO.png").
All errors:
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Could not open /home/ian/.julia/packages/ObjectDetector/sX9Lc/test/results/dog-cycle-car_nonsquare/v2_tiny_416_COCO.png for writing
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ArgumentError: Package ImageMagick [6218d12a-5da1-5696-b52f-db25d2ecc6d1] is required but does not seem to be installed:
 - Run `Pkg.instantiate()` to install all recorded dependencies.

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ArgumentError: Argument does not support conversion to png.
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Fatal error:
Pretrained Model: v2_tiny_416_COCO: Error During Test at /home/ian/.julia/packages/ObjectDetector/sX9Lc/test/maintests.jl:66
  Got exception outside of a @test
  Could not open /home/ian/.julia/packages/ObjectDetector/sX9Lc/test/results/dog-cycle-car_nonsquare/v2_tiny_416_COCO.png for writing
  Stacktrace: