Use `popen.signal.SIG*` constants instead of `os.execute('kill -%s %d')`

[Totktonada]

luatest/luatest/process.lua

Lines 94 to 102 in f8a1c10

	function Process.kill_pid(pid, signal, options)
	checks('number|string', '?number|string', {quiet = '?boolean'})
	-- Signal values are platform-dependent so we can not use ffi here
	signal = signal or 15
	local exit_code = os.execute('kill -' .. signal .. ' ' .. pid .. ' 2> /dev/null')
	if exit_code ~= 0 and not (options and options.quiet) then
	error('kill failed: ' .. exit_code)
	end
	end

Tarantool provides signal name to signal number mapping since the 2.4.1 version. We can use it if available and fallback to os.execute() on older versions.