6867244: Tests missing @run tag

Reviewed-by: sherman

jdk/test/java/nio/channels/DatagramChannel/BasicMulticastTests.java

@@ -25,6 +25,7 @@
  * @bug 4527345
  * @summary Unit test for DatagramChannel's multicast support
  * @build BasicMulticastTests NetworkConfiguration
+ * @run main BasicMulticastTests
  */
 
 import java.nio.ByteBuffer;

jdk/test/java/nio/channels/DatagramChannel/MulticastSendReceiveTests.java

@@ -25,6 +25,7 @@
  * @bug 4527345
  * @summary Unit test for DatagramChannel's multicast support
  * @build MulticastSendReceiveTests NetworkConfiguration
+ * @run main MulticastSendReceiveTests
  */
 
 import java.nio.ByteBuffer;

jdk/test/java/nio/file/Files/ContentType.java

@@ -26,6 +26,7 @@
  * @summary Unit test for probeContentType method
  * @library ..
  * @build ContentType SimpleFileTypeDetector
+ * @run main ContentType
  */
 
 import java.nio.file.*;