test: revert relsizev2 config (#11759)

## Problem

part of https://github.com/neondatabase/neon/issues/9516

One thing I realized in the past few months is that "no-way-back" things
like this are scary to roll out without a fine-grained rollout infra.
The plan was to flip the flag in the repo and roll it out soon, but I
don't think rolling out would happen in the near future. So I'd rather
revert the flag to avoid creating a discrepancy between staging and the
regress tests.

## Summary of changes

Not using rel_size_v2 by default in unit tests; we still have a few
tests to explicitly test the new format so we still get some test
coverages.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/fixtures/neon_fixtures.py

@@ -1279,7 +1279,8 @@ class NeonEnv:
                 )
 
             tenant_config = ps_cfg.setdefault("tenant_config", {})
-            tenant_config["rel_size_v2_enabled"] = True  # Enable relsize_v2 by default in tests
+            # This feature is pending rollout.
+            # tenant_config["rel_size_v2_enabled"] = True
 
             if self.pageserver_remote_storage is not None:
                 ps_cfg["remote_storage"] = remote_storage_to_toml_dict(

test_runner/regress/test_attach_tenant_config.py

@@ -186,7 +186,7 @@ def test_fully_custom_config(positive_env: NeonEnv):
             "type": "interpreted",
             "args": {"format": "bincode", "compression": {"zstd": {"level": 1}}},
         },
-        "rel_size_v2_enabled": False,  # test suite enables it by default as of https://github.com/neondatabase/neon/issues/11081, so, custom config means disabling it
+        "rel_size_v2_enabled": True,
         "gc_compaction_enabled": True,
         "gc_compaction_verification": False,
         "gc_compaction_initial_threshold_kb": 1024000,