Move the `write_to_app_pipe` that exists in python_tests in a common location

[hicklin]

The write_to_app_pipe function is replicated in a number of python tests. This should be moved into a common location, possibly made part of the MatterBaseTest class.

Consider requiring apps to provide an acknowledgement back when the command has been processes. This will allow for the removal of the current sleep(0.001) call that resolves the test flakiness.