test: dm: virtio_rng: Update virtio-rng test

The virtio-rng test to verify effective handling of oversized return
buffers checks that an (undocumented) error is raised, instead of the
real concern, which is the surrounding buffer integrity following a rng
function call.
Update the test to check that the other contents of a buffer remain
unchanged instead of looking for an error code.

Signed-off-by: Kavin Gunasekara <[email protected]>
Signed-off-by: Meet Patel <[email protected]>
Reviewed-by: Andre Przywara <[email protected]>

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