transport/grpc: deflake TestDialTCPUserTimeout
transport/grpc: deflake TestDialTCPUserTimeout

Supersedes CL https://code-review.googlesource.com/c/google-api-go-client/+/44273.

Change-Id: Iebd0762170df534fccfdde024933f9e48a82391f
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/44990
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,google/google-api-go-client