19889: Give sensitive test a prand sequence that passes.

19889: Give sensitive test a prand sequence that passes.

The previous version of the test relied on test input data,
superficially random but generated the same way each run using the
default-seeded global prand generator, that happens to avoid a bug
that would fail the test.

This change doesn't address the bug, it only fixes the problem of the
test starting to fail when something changes the pseudorandom input,
e.g., another unrelated module uses the math/rand package.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>