Running cdc_test binary fails with a segmentation fault
when run with --smp 1, because test_cdc_across_shards
assumes shard count to be >=2. This patch skips the test case
when run with a single shard and produces a log warning.
Message-Id: <9b00537db9419d8b7c545ce0c3b05b8285351e7d.1581600854.git.sarna@scylladb.com>