Fix memory leaks in unit tests

[nmuggli]

./configure --enable-debug --enable-sanitizer --enable-unit-tests
make -j libs
make unittests

[michaelrsweet]

This is a known ASAN "feature" on Linux; you need to set ASAN_OPTIONS to "leak_check_at_exit=false" in the environment to suppress these (bogus) memory leak reports since we don't care about them on exit - the OS reclaims all memory and explicitly freeing strings/arrays/etc. is both unnecessary and inefficient. Other platforms that implement the address sanitizer do not have this feature, BTW, just Linux...

If you look at the CI jobs you'll notice that we set the ASAN_OPTIONS environment variable for this reason.