test: fix flaky warning on attach (#4415)

added the `allowed_error` to the `positive_env` so any tests completing
the attach are allowed have this print out. they are allowed to do so,
because the `random_init_delay` can produce close to zero and thus the
first run will be near attach. Though... Unsure if we ever really need
the eviction task to run **before** it can evict something, as in after
20min or 24h.

in the failed test case however period is 20s so interesting that we
didn't run into this sooner.

evidence of flaky:
https://github.com/neondatabase/neon/actions/runs/5175677035/jobs/9323705929?pr=4399#step:4:38536

test_runner/regress/test_attach_tenant_config.py

@@ -20,6 +20,11 @@ def positive_env(neon_env_builder: NeonEnvBuilder) -> NeonEnv:
         test_name="test_attach_tenant_config",
     )
     env = neon_env_builder.init_start()
+
+    # eviction might be the first one after an attach to access the layers
+    env.pageserver.allowed_errors.append(
+        ".*unexpectedly on-demand downloading remote layer remote.* for task kind Eviction"
+    )
     assert isinstance(env.remote_storage, LocalFsStorage)
     return env