PR description:
> The test suite must always be defined (even when EVENT_SET_MEM_FUNCTIONS_IMPLEMENTED is not defined) so that the test harness doesn't fail due to not being able to find the raii_event_tests test.
>
> This improves upon 95f97f4 actually fixing https://github.com/bitcoin/bitcoin/issues/9493
Context from issue:
> core#9387 added a new test that uses libevent's event_set_mem_functions which is sometimes not included with libevent. In particular, Gentoo only enables this when libevent is installed with the "debug" option.
> Currently, this causes the build to simply fail on tests
This is a backport of core#16564