patch 8.2.0702: running channel tests may leave running process behind

Problem:    Running channel tests may leave running process behind.
Solution:   Make Python client exit when running into EOF. (Kurtis Rader,
            part of #6046)

src/testdir/test_channel_pipe.py

@@ -29,6 +29,8 @@ if __name__ == "__main__":
 
     while True:
         typed = sys.stdin.readline()
+        if typed == "":  # EOF -- stop
+            break
         if typed.startswith("quit"):
             print("Goodbye!")
             sys.stdout.flush()