Move the rtos stubs to rtos/tests/UNITTESTS

[rajkan01]

Description of defect

To move rtos stubs to rtos/tests/UNITTESTS directory so the stubs live with the associated component

Reference PR: #14773

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:

Could you add some more detail to the description? A good description should be at least 25 words.

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