[Perf] storage-blob ListBlobsTest setup fails if count==1

[mikeharder]

Perf test ListBlobsTest in package azure-storage-blob fails if count==1:

=== Options ===
Namespace(test='ListBlobsTest')
Namespace(parallel=1, duration=1, iterations=1, warmup=0, no_cleanup=False, sync=True,
max_put_size=67108864, max_block_size=4194304, buffer_threshold=4194305,
max_concurrency=1, size=10240, no_client_share=False, count=1)

=== Setup ===
Exception: Set of coroutines/Futures is empty.