Always define the raii_event_tests test suite

Summary:
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

Test Plan: ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9930