[v1.8.x] Backport "Explore behavior of flakey R test UNET from test_img_seg.R" #19186

[DickJC123]

Description

This is a backport of PR #19186 from v1.x to v1.8.x. With this PR's non-functional change to the UNET test of test_img_seg.R, the frequent but non-deterministic failure has not reoccurred. This suggests a race condition of some sort, but this has not been root-caused.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

@samskalicky

[mxnet-bot]

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