libbladeRF_test: Added common cmdline arg handling and device config

Most of the libbladeRF_test items contain their own command line
argument handling, yielding repeated code and inconsistent invocations.

This commit introduces common command-line argument handling and device
configuration for the most used device parameters.

Future patches will migrate the tests over to using this functionality.
New tests should use this immediately.
libbladeRF_test: Added frequency hopping test

This test is intended to help investigate reported issues involving
device lock-ups when changing frequency while samples are streaming.
libbladeRF_test: Addressed uninitialized variable warning (Clang)

'status' would not be initialized if t->num_iterations < 0. This shouldn't
happen anyway, but it has been fixed to keep the static analysis reports
free of false positives.