e2e: reduce worker threads during mock testing

We don't need much concurrency and having too many worker threads has
one disadvantage (besides resource usage): when the sidecar looses the
connection to the CSI driver, it calls klog.Fatal, which prints all
gouroutines. This can lead to much output.

test/e2e/testing-manifests/storage-csi/mock/csi-mock-driver.yaml

@@ -24,6 +24,9 @@ spec:
             - "-v=5"
             # Needed for fsGroup support.
             - "--default-fstype=ext4"
+            # We don't need much concurrency and having many gouroutines
+            # makes klog.Fatal during shutdown very long.
+            - "--worker-threads=5"
           env:
             - name: ADDRESS
               value: /csi/csi.sock