neon_local timeline import: create timelines on safekeepers (#12138)

neon_local's timeline import subcommand creates timelines manually, but
doesn't create them on the safekeepers. If a test then tries to open an
endpoint to read from the timeline, it will error in the new world with
`--timelines-onto-safekeepers`.

Therefore, if that flag is enabled, create the timelines on the
safekeepers.

Note that this import functionality is different from the fast import
feature (https://github.com/neondatabase/neon/issues/10188, #11801).

Part of #11670
As well as part of #11712

test_runner/regress/test_import.py

@@ -87,6 +87,9 @@ def test_import_from_vanilla(test_output_dir, pg_bin, vanilla_pg, neon_env_build
 
     # Set up pageserver for import
     neon_env_builder.enable_pageserver_remote_storage(RemoteStorageKind.LOCAL_FS)
+    neon_env_builder.storage_controller_config = {
+        "timelines_onto_safekeepers": True,
+    }
     env = neon_env_builder.init_start()
 
     env.pageserver.tenant_create(tenant)