Make sure tests run in kotlin sample

Author: dsyer

samples/grpc-server-kotlin/pom.xml

@@ -90,6 +90,7 @@
 
 	<build>
 		<sourceDirectory>src/main/kotlin</sourceDirectory>
+		<testSourceDirectory>src/test/kotlin</testSourceDirectory>
 		<plugins>
 			<plugin>
 				<groupId>org.jetbrains.kotlin</groupId>