Add cFE ES Child Task API functional tests

[skliper]

Is your feature request related to a problem? Please describe.
Need open source functional tests for certifiability

Describe the solution you'd like
Add cFE Child Task API functional tests
– CFE_ES_RegisterChildTask - Registers a cFE Child task associated with a cFE Application.
– CFE_ES_CreateChildTask - Creates a new task under an existing Application.
– CFE_ES_DeleteChildTask - Deletes a task under an existing Application.
– CFE_ES_ExitChildTask - Exits a child task.

Describe alternatives you've considered
N/A

Additional context
N/A

Requester Info
Jacob Hageman - NASA/GSFC