[unit-test] fix implicit conversion in test_lowpan.cpp (#4533)
[unit-test] fix implicit conversion in test_lowpan.cpp (#4533)

chshu/openthread,librasungirl/openthread,lanyuwen/openthread,jwhui/openthread,jwhui/openthread,bukepo/openthread,openthread/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,abtink/openthread,chshu/openthread,lanyuwen/openthread,lanyuwen/openthread,librasungirl/openthread,librasungirl/openthread,lanyuwen/openthread,chshu/openthread,jwhui/openthread,bukepo/openthread,bukepo/openthread,srickardti/openthread,chshu/openthread,librasungirl/openthread,lanyuwen/openthread,openthread/openthread,chshu/openthread,srickardti/openthread,bukepo/openthread,abtink/openthread,jwhui/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,openthread/openthread,openthread/openthread