(draft)tests: fix flaky TestGracefulShutdown

It seems that is the version not always got in Stdout, but in Stderr.

Added CombinedOutput (stdout+stderr) if exec.Cmd.Output() returned ""
while executing `tarantool --version`.

Closes #503

Author: laines-it

test_helpers/main.go

@@ -237,6 +237,11 @@ func IsTarantoolVersionLess(majorMin uint64, minorMin uint64, patchMin uint64) (
 
 	out, err := exec.Command(getTarantoolExec(), "--version").Output()
 
+	// It seems that is the version not always got in Stdout, but in Stderr
+	if string(out) == "" {
+		out, err = exec.Command(getTarantoolExec(), "--version").CombinedOutput()
+	}
+
 	if err != nil {
 		return true, err
 	}