Add missing test import (#23)

Motivation:

I noticed nightly-main failed because of a missing import in tests.
Auto-merge
blasted over this previously.

Modifications:

Add the missing import.

Result:

nightly-main passes.

Tests/GRPCReflectionServiceTests/GRPCReflectionServiceTests.swift

@@ -18,6 +18,7 @@ import Foundation
 import GRPCCore
 import GRPCInProcessTransport
 import GRPCReflectionService
+import SwiftProtobuf
 import Testing
 
 @Suite("gRPC Reflection Service Tests")