test: update test-run

Added the signal option into 'stop server' command.

How to use:

 | test_run:cmd('stop server foo with signal=KILL')

The 'stop server foo' command without the option sends SIGTERM as
before.

This feature is intended to be used in a fix of #4162 ('test:
gc.test.lua test fails on *.xlog files cleanup').

(cherry picked from commit 71f7ecf)