Always define the raii_event_tests test suite
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:
> [[https://github.com/bitcoin/bitcoin/pull/9387 | 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 [[https://github.com/bitcoin/bitcoin/pull/16564 | core#16564]]

Test Plan: `ninja check`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

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

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