unflake TestReadWriteTransaction_ErrorReturned
unflake TestReadWriteTransaction_ErrorReturned

The mehod shouldHaveReceived was called twice
inside the test case, but calling this method
drains the requests from the mock server, which
will cause the second call to always return an
empty range of requests.

Fixes #1409

Change-Id: I25e07d787f206b07554de36e045c033c128e75ca
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/40410
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go