FPGA CI TEST SHIELD: add test config json file

[jeromecoutant]

Summary of changes

Discussion started in #12615

Impact of changes

As soon as this PR is merged, @jamesbeyond will push a ticket to update
CI scripts repo to use this new fpga.json

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

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[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 4 of 4 test jobs passed
Build number : 1
Build artifacts

[jeromecoutant]

@jamesbeyond
Time to push a ticket to update CI scripts repo to use this new fpga.json :-)

[jeromecoutant]

@jamesbeyond
Time to push a ticket to update CI scripts repo to use this new fpga.json :-)

If done, please restart #12615 CI ?

Thx