IOTSTOR-978: Skip a unstable testcase until fixed

[SeppoTakalo]

Description (required)

Skip a unstable testcase until fixed.

Work ongoing to find the root cause, but in the mean time, we could unblock the master by skipping the testcase.

Summary of change (What the change is for and why)

Documentation (Details of any document updates required)

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Pull request type (required)

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[ ] Feature update (New feature / Functionality change / New API)
[ ] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results (required)

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers (optional)

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

@SeppoTakalo, thank you for your changes.
@ARMmbed/mbed-os-storage @ARMmbed/mbed-os-maintainers please review.

[adbridge]

@SeppoTakalo this is a test update so I've updated the PR type to Patch

[adbridge]

Looks sensible to me AFAICT

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 5 of 5 test jobs passed
Build number : 1
Build artifacts