Exclude 'flatten' file from coverage by default

tarantool/crud produce 'flatten' file while flattening. 

```lua
crud.insert_object('space', obj)
```

```bash
.rocks/bin/luatest integration_api --coverage
.rocks/bin/luacov .
Couldn't open flatten: No such file or directory
```

I propose to exclude it from coverage collection by default:

```lua
exclude = {
	'flatten',
}
```