random_test is very slow since isc_random change

Before change 4947:

$ kyua test random_test
random_test:isc_rng_binarymatrixrank_16  ->  passed  [1.503s]
random_test:isc_rng_binarymatrixrank_bytes  ->  passed  [0.918s]
random_test:isc_rng_blockfrequency_16  ->  passed  [0.718s]
random_test:isc_rng_blockfrequency_bytes  ->  passed  [0.134s]
random_test:isc_rng_monobit_16  ->  passed  [0.706s]
random_test:isc_rng_monobit_bytes  ->  passed  [0.130s]
random_test:isc_rng_runs_16  ->  passed  [1.151s]
random_test:isc_rng_runs_bytes  ->  passed  [0.580s]

After change 4947:

$ kyua test random_test
random_test:isc_random_binarymatrixrank_16  ->  passed  [57.969s]
random_test:isc_random_binarymatrixrank_bytes  ->  passed  [4.127s]
random_test:isc_random_blockfrequency_16  ->  passed  [58.222s]
random_test:isc_random_blockfrequency_bytes  ->  passed  [3.374s]
random_test:isc_random_monobit_16  ->  passed  [59.008s]
random_test:isc_random_monobit_bytes  ->  passed  [3.351s]
random_test:isc_random_runs_16  ->  passed  [59.605s]
random_test:isc_random_runs_bytes  ->  passed  [3.649s]

Even worse, when running a full "make unit" with other unit tests being run in parallel, these are the grepped-out lines for random_test:

random_test:isc_random_binarymatrixrank_bytes  ->  passed  [16.648s]
random_test:isc_random_monobit_bytes  ->  passed  [15.136s]
random_test:isc_random_blockfrequency_bytes  ->  passed  [25.718s]
random_test:isc_random_runs_bytes  ->  passed  [18.061s]
random_test:isc_random_monobit_16  ->  passed  [171.179s]
random_test:isc_random_binarymatrixrank_16  ->  passed  [177.380s]
random_test:isc_random_blockfrequency_16  ->  passed  [176.568s]
random_test:isc_random_runs_16  ->  passed  [166.775s]

Of greatest concern: the unit test doesn't appear to have been significantly changed, other than switching to the new isc_random API. It isn't running more iterations or anything. This suggests that isc_random performs less than a tenth as well as isc_rng did, which is alarming. If true, I'd expect it to impact on resolver performance.