[feature-wisun] Fix feature-wisun tests

[artokin]

Summary of changes

feature-wisun test are failing because of error:
test.py: error: argument --app-config: TESTS/configs/fpga.json does not exist in the filesystem.

Include missing file to the feature-wisun branch

Impact of changes

Migration actions required

Documentation

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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

@ARMmbed/mbed-os-mesh

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[ciarmcom]

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

[0xc0170]

Will schedule a CI job later today