TEST: fix a bug in ATCmdParser test

[jamesbeyond]

Summary of changes

This fix the bug in ATCmdParser test, where the number of arguments does not match with the format specifier. This caused a segfault on Mac OS.

and this is reported in #12346

Impact of changes

Migration actions required

Documentation

Documents not needed

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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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[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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