tests: enable wal reader fanout in tests (#10301)

Note: this has to merge after the release is cut on `2025-01-17` for
compat tests to start passing.

## Problem

SK wal reader fan-out is not enabled in tests by default.

## Summary of changes

Enable it.

test_runner/fixtures/neon_fixtures.py

@@ -4390,6 +4390,7 @@ class Safekeeper(LogUtils):
                 "1s",
                 "--eviction-min-resident",
                 "10s",
+                "--wal-reader-fanout",
             ]
 
         self.extra_opts = extra_opts