tests: add a log allow list to stabilize benchmarks (#7251)

## Problem

#7227 destabilized various
tests in the performance suite, with log errors during shutdown. It's
because we switched shutdown order to stop the storage controller before
the pageservers.

## Summary of changes

- Tolerate "connection failed" errors from pageservers trying to
validation their deletion queue.

test_runner/fixtures/pageserver/allowed_errors.py

@@ -86,6 +86,9 @@ def scan_pageserver_log_for_errors(
     # This is especially pronounced in tests that set small checkpoint
     # distances.
     ".*Flushed oversized open layer with size.*",
+    # During teardown, we stop the storage controller before the pageservers, so pageservers
+    # can experience connection errors doing background deletion queue work.
+    ".*WARN deletion backend: calling control plane generation validation API failed.*Connection refused.*",
 )