Improve CPU usage in tests with IO threads

[uriyage]

Currently, when running tests with IO threads, we set the events-per-io-thread config to 0. This activated IO threads 100% of the time, regardless of the number of IO events.

This is causing issues with tests running multiple server instances, as it drained machine CPU resources. As a result, tests could have very long runtimes, especially on limited instances.
For example, in https://github.com/valkey-io/valkey/actions/runs/10066315827/job/27827426986?pr=804, the Cluster consistency during live resharding test ran for 1 hour and 41 minutes.

This PR addresses the issue by:

1. Deactivating IO threads when there are no IO events
2. Continuing to offload all IO events to IO threads

Tested on 16 cores instance, after implementing these changes, the runtime for the Cluster consistency during live resharding test dropped from 7 minutes an 14 seconds to 3 minutes and 28 seconds.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.35%. Comparing base (59aa008) to head (f17fd39).
Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #823      +/-   ##
============================================
+ Coverage     70.18%   70.35%   +0.16%     
============================================
  Files           112      112              
  Lines         61311    61310       -1     
============================================
+ Hits          43031    43132     +101     
+ Misses        18280    18178     -102     

Files	Coverage Δ
src/io_threads.c	7.90% <0.00%> (+0.03%)	⬆️

... and 7 files with indirect coverage changes