tests: fix a flake in test_sharding_split_compaction (#8136)

## Problem

This test could occasionally trigger a "removing local file ... because
it has unexpected length log" when using the
`compact-shard-ancestors-persistent` failpoint is in use, which is
unexpected because that failpoint stops the process when the remote
metadata is in sync with local files.

It was because there are two shards on the same pageserver, and while
the one being compacted explicitly stops at the failpoint, another shard
was compacting in the background and failing at an unclean point. The
test intends to disable background compaction, but was mistakenly
revoking the value of `compaction_period` when it updated
`pitr_interval`.

Example failure:

https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8123/9602976462/index.html#/testresult/7dd6165da7daef40

## Summary of changes

- Update `TENANT_CONF` in the test to use properly typed values, so that
it is usable in pageserver APIs as well as via neon_local.
- When updating tenant config with `pitr_interval`, retain the overrides
from the start of the test, so that there won't be any background
compaction going on during the test.

test_runner/regress/test_sharding.py

@@ -190,19 +190,20 @@ def test_sharding_split_compaction(neon_env_builder: NeonEnvBuilder, failpoint:
     """
     Test that after a split, we clean up parent layer data in the child shards via compaction.
     """
+
     TENANT_CONF = {
         # small checkpointing and compaction targets to ensure we generate many upload operations
-        "checkpoint_distance": f"{128 * 1024}",
-        "compaction_threshold": "1",
-        "compaction_target_size": f"{128 * 1024}",
+        "checkpoint_distance": 128 * 1024,
+        "compaction_threshold": 1,
+        "compaction_target_size": 128 * 1024,
         # no PITR horizon, we specify the horizon when we request on-demand GC
         "pitr_interval": "3600s",
         # disable background compaction and GC. We invoke it manually when we want it to happen.
         "gc_period": "0s",
         "compaction_period": "0s",
         # create image layers eagerly, so that GC can remove some layers
-        "image_creation_threshold": "1",
-        "image_layer_creation_check_threshold": "0",
+        "image_creation_threshold": 1,
+        "image_layer_creation_check_threshold": 0,
     }
 
     neon_env_builder.storage_controller_config = {
@@ -261,7 +262,9 @@ def test_sharding_split_compaction(neon_env_builder: NeonEnvBuilder, failpoint:
     env.pageserver.start()
 
     # Cleanup part 2: once layers are outside the PITR window, they will be rewritten if they are partially redundant
-    env.storage_controller.pageserver_api().set_tenant_config(tenant_id, {"pitr_interval": "0s"})
+    updated_conf = TENANT_CONF.copy()
+    updated_conf["pitr_interval"] = "0s"
+    env.storage_controller.pageserver_api().set_tenant_config(tenant_id, updated_conf)
     env.storage_controller.reconcile_until_idle()
 
     for shard in shards: