Refactor connectivity cellular unit test to use mocks lib what it depends

[rajkan01]

Description of defect

Refactor connectivity cellular unit tests to use mocks library what is depends

Target(s) affected by this defect ?

N/A

Toolchain(s) (name and version) displaying this defect ?

N/A

What version of Mbed-os are you using (tag or sha) ?

Latest

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

N/A

How is this defect reproduced ?

N/A

[mbedmain]

@rajkan01 thank you for raising this issue.Please take a look at the following comments:

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