tests: adjust threshold in test_partial_evict_tenant (#8509)

## Problem

This test was destabilized by
https://github.com/neondatabase/neon/pull/8431. The threshold is
arbitrary & failures are still quite close to it. At a high level the
test is asserting "eviction was approximately fair to these tenants",
which appears to still be the case when the abs diff between ratios is
slightly higher at ~0.6-0.7.

## Summary of changes

- Change threshold from 0.06 to 0.065. Based on the last ~10 failures
that should be sufficient.

test_runner/regress/test_disk_usage_eviction.py

@@ -591,7 +591,7 @@ def test_partial_evict_tenant(eviction_env: EvictionEnv, order: EvictionOrder):
         abs_diff = abs(ratio - expected_ratio)
         assert original_count > count_now
 
-        expectation = 0.06
+        expectation = 0.065
         log.info(
             f"tenant {tenant_id} layer count {original_count} -> {count_now}, ratio: {ratio}, expecting {abs_diff} < {expectation}"
         )