cli: Ignore libevent warnings

Summary:
Should fix rpc tests that fail due to an unclean stderr.

Backport of Bitcoin Core PR13394
https://github.com/bitcoin/bitcoin/pull/13394

Test Plan:
In a macOS terminal:

./src/bitcoind -regtest

In another macOS terminal:

./src/bitcoin-cli -regtest help

The following warning should not be seen in stdout/stderr:
[warn] kq_init: detected broken kqueue; not using.: Undefined error: 0

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4005