ci: bluetooth-tests: Run with GNU parallel

This commit updates the bluetooth-tests workflow to install the GNU
parallel utility to enable parallel execution of the Bluetooth tests.

Note that the Bluetooth test script automatically parallelises the
tests when `parallel` is available in the PATH; otherwise, it
sequentially executes the tests.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>