Fix some memory leaks in tests

[rburing]

This fixes some memory leaks in tests which were relatively easy to track down. This is not really important by itself (and it doesn't affect user code), but it will allow focusing on the more interesting leaks.

Maybe in the future CI could check for leaks (though it may have a big resource cost).

[fredrik-johansson]

Thanks!

Maybe in the future CI could check for leaks (though it may have a big resource cost).

It should be possible with a really small FLINT_TEST_MULTIPLIER and perhaps explicitly disabling certain slow tests.