tests/qtest/npcm7xx_rng-test: Disable randomness tests

The randomness tests in the NPCM7xx RNG test fail intermittently
but fairly frequently. On my machine running the test in a loop:
 while QTEST_QEMU_BINARY=./qemu-system-aarch64 ./tests/qtest/npcm7xx_rng-test; do true; done

will fail in less than a minute with an error like:
ERROR:../../tests/qtest/npcm7xx_rng-test.c:256:test_first_byte_runs:
assertion failed (calc_runs_p(buf.l, sizeof(buf) * BITS_PER_BYTE) > 0.01): (0.00286205989 > 0.01)

(Failures have been observed on all 4 of the randomness tests,
not just first_byte_runs.)

It's not clear why these tests are failing like this, but intermittent
failures make CI and merge testing awkward, so disable running them
unless a developer specifically sets QEMU_TEST_FLAKY_RNG_TESTS when
running the test suite, until we work out the cause.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201102152454.8287-1-peter.maydell@linaro.org
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>

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