A few more initializer fixes.

Sources/gRPC/GenCodeSupport/ClientCallBidirectionalStreaming.swift

@@ -94,6 +94,8 @@ open class ClientCallBidirectionalStreamingTestStub<InputType: Message, OutputTy
 
   open var inputs: [InputType] = []
   open var outputs: [OutputType] = []
+  
+  public init() {}
 
   open func receive(completion: @escaping (OutputType?, ClientError?) -> Void) throws {
     if let output = outputs.first {

Sources/gRPC/GenCodeSupport/ClientCallClientStreaming.swift

@@ -86,6 +86,8 @@ open class ClientCallClientStreamingTestStub<InputType: Message, OutputType: Mes
 
   open var inputs: [InputType] = []
   open var output: OutputType?
+  
+  public init() {}
 
   open func send(_ message: InputType, errorHandler _: @escaping (Error) -> Void) throws {
     inputs.append(message)

Sources/gRPC/GenCodeSupport/ClientCallServerStreaming.swift

@@ -81,6 +81,8 @@ open class ClientCallServerStreamingTestStub<OutputType: Message>: ClientCallSer
   open class var method: String { fatalError("needs to be overridden") }
 
   open var outputs: [OutputType] = []
+  
+  public init() {}
 
   open func receive(completion: @escaping (OutputType?, ClientError?) -> Void) throws {
     if let output = outputs.first {

Sources/gRPC/GenCodeSupport/ServiceClient.swift

@@ -71,4 +71,6 @@ open class ServiceClientTestStubBase: ServiceClient {
   open var metadata = Metadata()
   open var host = ""
   open var timeout: TimeInterval = 0
+
+  public init() {}
 }