fix(cmake): support CMock subdir option with idf_component_mock (IDFGH-14547)

[BastienGermond]

Fixes #13360.

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[KonstantinKondrashov]

sha=1c78007cafa2c3a34021bca77abb30c4941b3e06

[BastienGermond]

Hey @KonstantinKondrashov, thanks for taking up this PR. Is there anything blocking the merge ? Let me know if I can help.