Test case memory leak from Happy Families

While working on fixing CI, the following showed up:

routerkeys/load_family_keys: [forking] 
=================================================================
==17751==ERROR: LeakSanitizer: detected memory leaks
Direct leak of 96 byte(s) in 1 object(s) allocated from:
    #0 0x564b5b561add in malloc (/builds/tpo/core/tor/build/src/test/test+0x59aadd)
    #1 0x564b5be401ce in tor_malloc_ /builds/tpo/core/tor/src/lib/malloc/malloc.c:45:12
    #2 0x564b5be401ce in tor_malloc_zero_ /builds/tpo/core/tor/src/lib/malloc/malloc.c:71:18
    #3 0x564b5bc8c7a3 in load_family_id_keys_impl /builds/tpo/core/tor/src/feature/relay/routerkeys.c:818:14
    #4 0x564b5b9c3705 in test_routerkeys_load_family_keys /builds/tpo/core/tor/src/test/test_routerkeys.c:811:3
    #5 0x564b5baabf2e in testcase_run_bare_ /builds/tpo/core/tor/src/ext/tinytest.c:107:2
    #6 0x564b5baabf2e in testcase_run_forked_ /builds/tpo/core/tor/src/ext/tinytest.c:201:12
    #7 0x564b5baabf2e in testcase_run_one /builds/tpo/core/tor/src/ext/tinytest.c:267:13
    #8 0x564b5baac743 in tinytest_main /builds/tpo/core/tor/src/ext/tinytest.c:454:5
    #9 0x564b5baaa43a in main /builds/tpo/core/tor/src/test/testing_common.c:424:22
    #10 0x7fdfdc2c0d79 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x23d79)
Direct leak of 96 byte(s) in 1 object(s) allocated from:
    #0 0x564b5b561add in malloc (/builds/tpo/core/tor/build/src/test/test+0x59aadd)
    #1 0x564b5be401ce in tor_malloc_ /builds/tpo/core/tor/src/lib/malloc/malloc.c:45:12
    #2 0x564b5be401ce in tor_malloc_zero_ /builds/tpo/core/tor/src/lib/malloc/malloc.c:71:18
    #3 0x564b5bc8c7a3 in load_family_id_keys_impl /builds/tpo/core/tor/src/feature/relay/routerkeys.c:818:14
    #4 0x564b5b9c3573 in test_routerkeys_load_family_keys /builds/tpo/core/tor/src/test/test_routerkeys.c:803:3
    #5 0x564b5baabf2e in testcase_run_bare_ /builds/tpo/core/tor/src/ext/tinytest.c:107:2
    #6 0x564b5baabf2e in testcase_run_forked_ /builds/tpo/core/tor/src/ext/tinytest.c:201:12
    #7 0x564b5baabf2e in testcase_run_one /builds/tpo/core/tor/src/ext/tinytest.c:267:13
    #8 0x564b5baac743 in tinytest_main /builds/tpo/core/tor/src/ext/tinytest.c:454:5
    #9 0x564b5baaa43a in main /builds/tpo/core/tor/src/test/testing_common.c:424:22
    #10 0x7fdfdc2c0d79 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x23d79)
SUMMARY: AddressSanitizer: 192 byte(s) leaked in 2 allocation(s).
[did not exit cleanly.]
  [load_family_keys FAILED]

See: https://gitlab.torproject.org/tpo/core/tor/-/jobs/899853