tests/qtest/npcm7xx_rng-test: dump random data on failure

tests/qtest/npcm7xx_rng-test: dump random data on failure

Dump the collected random data after a randomness test failure.

Note that this relies on the test having called
g_test_set_nonfatal_assertions() so we don't abort immediately on the
assertion failure.

Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: minor commit message tweak]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>