Fix broken pipe from gdb after killing process

Author: Sofia Kopikova

testgres/plugins/pg_probackup2/pg_probackup2/gdb.py

@@ -102,9 +102,8 @@ def get_line(self):
         return line
 
     def kill(self):
+        self._execute('kill')
         self._did_quit = True
-        self.proc.kill()
-        self.proc.wait(3)
         self.proc.stdin.close()
         self.proc.stdout.close()