trace: fix TestGRPCInterceptor
trace: fix TestGRPCInterceptor

Add grpc.WithBlock to Dial of local server.

Something in gRPC doesn't like asynchronous dials of in-process
servers.

Change-Id: Ia0f4ca426003bd577112a0ce4a5b3f3ef04cdc53
Reviewed-on: https://code-review.googlesource.com/18171
Reviewed-by: JBD <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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