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)