test_proxy_steerable: reduce cpu and thread count

[nnog]

Problem: test_proxy_steerable flakiness on slow CI VMs

The test is flacky and fails on slow architectures:

[ 2361s] FAIL: tests/test_proxy_steerable [ 2361s] ================================ [ 2361s] [ 2361s] tests/test_proxy_steerable.cpp:458:test_proxy_steerable:FAIL: Expression Evaluated To FALSE

https://build.opensuse.org/build/network:messaging:zeromq:git-stable/Fedora_Rawhide/s390x/libzmq/_log

Originally posted by @bluca in https://github.com/zeromq/libzmq/issues/4600#issuecomment-1760402811

Solution: fewer worker threads, poll rather than thrash, consistent timings. user cpu ~300% -> 15%.