sw test tool: Optimize similar methods

 - _test_flow_install()
 - _test_meter_install()

are merged as

 - _test_msg_install()

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
sw test tool: Remove unused codes

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
sw test tool: pylint

W:546,14: Access to a protected member _send_msg of a client class (protected-access)

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>