Use no test fixture for the RCU unit tests

Summary:
So far the test fixture makes no use of any RCU pointer, but the test will break otherwise as it as expectations on the cleanup count.
Since the fixture is unused, the most simple solution is to drop it.

Test Plan:

ninja check-bitcoin-rcu_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12917