Fix snapshot_read_kill_operations.js to handle read-only transactions that are aborted

It turns out that for read-only transactions, it is allowable that one shard commit and one shard abort, as long as the client receives an error (since "committing" for a read-only transaction doesn't do anything except wait for the data read to be majority committed). In this case, a read-only transaction was interrupted, causing one shard to commit and the other to abort. The test should be fixed to handle this case.