fix stalled subprocess close

It happens from time to time that subprocess close() stalls forever. I guess the
reason is a raise conditions when terminating the process and closing the piped
streams.

A note in Python's subprocess documentation[1] suggests to use following code
snipped to properly terminate/kill a subprocess:

    proc = subprocess.Popen(...)
    try:
        outs, errs = proc.communicate(timeout=15)
    except TimeoutExpired:
        proc.kill()
        outs, errs = proc.communicate()

Reference:
[1]: https://docs.python.org/3/library/subprocess.html

tbot/machine/channel/subprocess.py

@@ -104,7 +104,11 @@ def close(self) -> None:
         self.p.terminate()
         os.close(self.pty_slave)
         os.close(self.pty_master)
-        self.p.wait()
+        try:
+            outs, errs = self.p.communicate(timeout=10)
+        except subprocess.TimeoutExpired:
+            self.p.kill()
+            outs, errs = self.p.communicate()
 
         # Wait for all processes in the session to end.  Most of the time
         # this will return immediately, but in some cases (eg. a serial session