elements_dtls test hangs sometimes

The following discussion from !1573 (merged) should be addressed:

@heftig started a discussion: (+2 comments)
Actually, I just noticed the run after I applied this passed because the dtls test only took 0.86s:
```
76/88 elements_dtls                  OK             0.86s
```
Yeah, this one is just flaky.
```
76/88 elements_dtls                  FAIL           60.71s (exit status 1)
```