tests: Increase timeout in test_create_churn_during_restart (#9736)

This test was seen to be flaky, e.g. at:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9457/11804246485/index.html#suites/ec4311502db344eee91f1354e9dc839b/982bd121ea698414/.
If I _reduce_ the timeout from 10s to 8s on my laptop, it reliably hits
that timeout and fails. That suggests that the test is pretty close to
the edge even when it passes. Let's bump up the timeout to 30 s to make
it more robust.

See also #9730, although the
error message is different there.

test_runner/regress/test_tenants.py

@@ -427,7 +427,7 @@ def create_bg(delay_ms):
             env.pageserver.start()
 
             for f in futs:
-                f.result(timeout=10)
+                f.result(timeout=30)
 
     # The tenant should end up active
     wait_until_tenant_active(env.pageserver.http_client(), tenant_id, iterations=10, period=1)