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').

test-run @ c2c4f061

-Subproject commit 89808d60eb3b5130e227fc1a7866f2ad5a197bea
+Subproject commit c2c4f061207ebcccb83f0a9c330f6ddadabfab5d