test: fix flaky box/func_reload test

This commit fixes the following failure:

```
--- box/func_reload.result        Fri Dec  3 15:15:05 2021
+++ var/rejects/box/func_reload.reject    Fri Dec  3 15:15:12 2021
@@ -202,11 +202,11 @@
 ...
 ch:get()
 ---
+- [[2]]
+...
+ch:get()
+---
 - [[1]]
-...
-ch:get()
----
-- [[2]]
 ...
 s:drop()
 ---
```

The function reloaded in this test yields, so there's no guarantee that
the first reload completes before the second one. Fix this by discarding
`ch:get()` result.

Closes https://github.com/tarantool/tarantool-qa/issues/15