test: idempotent stop in helpers

Before this patch, calling StopTarantool wasn't idempotent because it
accepts a struct copy and doesn't actually set Cmd to nil. Setting
Cmd.Process to nil is effective since it's a pointer. Reworking helpers
to use pointer would be better, but it would break existing API.

test_helpers/main.go

@@ -293,7 +293,7 @@ func StopTarantool(inst TarantoolInstance) {
 			log.Fatalf("Failed to wait for Tarantool process to exit, got %s", err)
 		}
 
-		inst.Cmd = nil
+		inst.Cmd.Process = nil
 	}
 }