Flakey cpu R test?

[DickJC123]

Description

I think it's important to document non-deterministic CI failures we've run into during our PR polishing. Along that vein, I saw the following error in pipline https://jenkins.mxnet-ci.amazon-ml.com/blue/rest/organizations/jenkins/pipelines/mxnet-validation/pipelines/unix-cpu/branches/PR-19148/runs/2/nodes/303/steps/771/log/?start=0

[2020-09-17T07:57:51.701Z] Loading required package: testthat
[2020-09-17T07:57:51.956Z] v |  OK F W S | Context
[2020-09-17T07:57:56.111Z] trying URL 'https://apache-mxnet.s3-accelerate.dualstack.amazonaws.com/R/data/ISBI.zip'
[2020-09-17T07:57:56.111Z] Content type 'application/zip' length 25275513 bytes (24.1 MB)
[2020-09-17T07:57:56.111Z] ==================================================
[2020-09-17T07:57:56.111Z] downloaded 24.1 MB
[2020-09-17T07:57:56.111Z] 
[2020-09-17T07:57:58.622Z] 
/ |   0       | Image segmentation
- |   0 1     | Image segmentation
x |   0 1     | Image segmentation [3.2 s]
[2020-09-17T07:57:58.622Z] --------------------------------------------------------------------------------
[2020-09-17T07:57:58.622Z] test_img_seg.R:157: error: UNET
[2020-09-17T07:57:58.622Z] could not find function "mx.ctx.default"
[2020-09-17T07:57:58.622Z] --------------------------------------------------------------------------------

A retry of the pipeline passed without this error.

[DickJC123]

Similar failure hitting another PR:

https://jenkins.mxnet-ci.amazon-ml.com/blue/rest/organizations/jenkins/pipelines/mxnet-validation/pipelines/unix-cpu/branches/PR-19142/runs/7/nodes/303/steps/697/log/?start=0

[DickJC123]

Resurfacing of error from years ago: #13439

[DickJC123]

Hitting another PR: https://jenkins.mxnet-ci.amazon-ml.com/blue/rest/organizations/jenkins/pipelines/mxnet-validation/pipelines/unix-cpu/branches/PR-19184/runs/3/nodes/303/steps/781/log/?start=0

[kpuatamazon]

I'm seeing it too in #19201 https://jenkins.mxnet-ci.amazon-ml.com/blue/rest/organizations/jenkins/pipelines/mxnet-validation/pipelines/unix-cpu/branches/PR-19201/runs/1/nodes/303/steps/764/log/?start=0 ironically trying to fix my flakey test.

[DickJC123]

I believe now-merged PR #19186 fixes this issue.