test(perf): return number of bytes written in _DummyListBuffer.write

Also fixed a pre-existing lint error (unused import io.BytesIO).

Author: zhixiangli

packages/google-cloud-storage/tests/perf/microbenchmarks/time_based/reads/test_reads.py

@@ -19,7 +19,6 @@
 import os
 import random
 import time
-from io import BytesIO
 
 import pytest
 
@@ -126,7 +125,9 @@ def __init__(self):
 
     def write(self, data):
         self.chunks.append(data)
-        self.size += len(data)
+        nbytes = len(data)
+        self.size += nbytes
+        return nbytes
 
     def getvalue(self):
         return b"".join(self.chunks)