Fix several memory leaks.
The libkqueue tests have several places that leak memory by using an
Rework to save the pointer returned from kevent_to_str() and then
free() it after it has been used.
Reported by: asomers (pointer to Coverity), Coverity
CID: 1296063, 1296064, 1296065, 1296066, 1296067, 1350287, 1394960
Sponsored by: Dell EMC