Break out of loop on EOF in asyncio echo test programs.

Lib/test/test_asyncio/echo.py

@@ -3,4 +3,6 @@ import os
 if __name__ == '__main__':
     while True:
         buf = os.read(0, 1024)
+        if not buf:
+            break
         os.write(1, buf)

Lib/test/test_asyncio/echo3.py

@@ -3,6 +3,8 @@ import os
 if __name__ == '__main__':
     while True:
         buf = os.read(0, 1024)
+        if not buf:
+            break
         try:
             os.write(1, b'OUT:'+buf)
         except OSError as ex: