Sleep in test_scrubber_physical_gc (#8798)

This copies a piece of code from `test_scrubber_physical_gc_ancestors`
to fix a source of flakiness: later on we rely on stuff being older than
a second, but the test can run faster under optimal conditions (as
happened to me locally, but also obvservable in
[this](https://neon-github-public-dev.s3.amazonaws.com/reports/main/10470762360/index.html#testresult/f713b02657db4b4c/retries)
allure report):

```
test_runner/regress/test_storage_scrubber.py:169: in test_scrubber_physical_gc
    assert gc_summary["remote_storage_errors"] == 0
E   assert 1 == 0
```

test_runner/regress/test_storage_scrubber.py

@@ -152,6 +152,9 @@ def test_scrubber_physical_gc(neon_env_builder: NeonEnvBuilder, shard_count: Opt
         # This write includes remote upload, will generate an index in this generation
         workload.write_rows(1)
 
+    # We will use a min_age_secs=1 threshold for deletion, let it pass
+    time.sleep(2)
+
     # With a high min_age, the scrubber should decline to delete anything
     gc_summary = env.storage_scrubber.pageserver_physical_gc(min_age_secs=3600)
     assert gc_summary["remote_storage_errors"] == 0