test-mktemp: plug memory and descriptor leaks

We test xmkstemp() in our helper by just calling: xmkstemp(xstrdup(argv[1])); This leaks both the copied string as well as the descriptor returned by the function. In practice this isn't a big deal, since we immediately exit the program, but: 1. LSan will complain about the memory leak. The only reason we did not notice this in our leak-checking builds is that both of the callers in the test suite (both in t0070) pass a broken template (and expect failure). So the function calls die() before we can actually leak. But it's an accident waiting to happen if anybody adds a call which succeeds. 2. Coverity complains about the descriptor leak. There's a long list of uninteresting or false positives in Coverity's results, but since we're here we might as well fix it, too. I didn't bother adding a new test that triggers the leak. It's not even in real production code, but just in the test-helper itself. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>