[feature-wisun] Update cert for testing server

[mikaleppanen]

Summary of changes

Testing server cert has changed, this uses the new cert.

This is feature-wisun version of #14173

Impact of changes

None

Migration actions required

None

Documentation

None

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Pull request type

[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

[] 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

@mikter @artokin

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[mikaleppanen]

@0xc0170 can this proceed to testing? It is blocking other feature-wisun pull requests.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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