big_decimal: Add more tests
big_decimal: Add more tests

It looks like an order version of my patch series was merged. The only
difference is that the new one had more tests. This patch adds the
missing ones.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200630141150.1286893-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla