libbladeRF_test_ctrl: Error out on invalid test name
libbladeRF_test_ctrl: Removed use of (s)rand()

Using a quick and dirty xorshift() implementation to more deterministic
behavior across systems.
libbladeRF_test_ctrl: Removed samplerate test from threaded test

This has potential to cause chaos since we're running a full duplex
stream, and changing sample rates to values possibly way above
what the system supports.
libbladeRF_test_ctrl: Increased timeout
libbladeRF_test_ctrl: Print error message on RX/TX errors
libbladeRF_test_ctrl: Restore sample rate to "default" value