[Thread] tests a single ThreadNetworkController

Simplifies the CTS tests by testing only the first controller given
there is only one controller. No testing logic changes.

This commit also moves the helper methods to end of the file as they are
not the focus.

Bug: 321662734
Test: m CtsThreadNetworkTestCases
Change-Id: I4acf26e13ee112df2b7378b1005a1a14de4d783e