Race condition between FFMpegWriter and ImageWriter tests

[Ninpo]

It's possible when tests are run in parallel for the FFmpegWriter:Gif and ImageWriter:Gif tests to collide due to creation of the same file, output1.gif.

Discovered over at Gentoo Linux during version bump to 0.3.2: https://bugs.gentoo.org/909759

PR submitted with suggested fix.

[stale[bot]]

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