nsfarm/cli: add dedicated method to send ctrl+c character

This is only alias for sending ^C but this way it is way less cryptic in
tests them self.

nsfarm/cli.py

@@ -166,6 +166,11 @@ class Shell(Cli):
             raise Exception("Command probably won't fit to terminal. Split it or increase number of columns.")
         return super().command(cmd)
 
+    def ctrl_c(self):
+        """Sends ^C character.
+        """
+        self._sh.send('\x03')
+
     def file_read(self, path):
         """Read file trough shell.
 

tests/conftest.py

@@ -142,8 +142,7 @@ def fixture_client_board(board, board_serial, board_root_password, lan1_client):
     yield nsfarm.cli.Shell(pexp, flush=False)  # TODO drop this flush disable when it works
 
     board.set_serial_flush(False)
-    # Kill tail -f on serial console
-    board_serial.send('\x03')
+    board_serial.ctrl_c()  # Terminate tail -f on serial console
     board_serial.prompt()