test: adjust checkpoint distance in test_layer_map (#6842)

https://github.com/neondatabase/neon/commit/587cb705b898565d459d044df84d1ac2633f00bf
changed the layer rolling logic to more closely obey the
`checkpoint_distance` config. Previously, this test was getting
layers significantly larger than the 8K it was asking for. Now the
payload in the layers is closer to 8K (which means more layers in
total).

Tweak the `checkpoint_distance` to get a number of layers more
reasonable for this test. Note that we still get more layers than
before (~8K vs ~5K).

test_runner/performance/test_layer_map.py

@@ -17,10 +17,10 @@ def test_layer_map(neon_env_builder: NeonEnvBuilder, zenbenchmark):
     tenant, _ = env.neon_cli.create_tenant(
         conf={
             "gc_period": "0s",
-            "checkpoint_distance": "8192",
+            "checkpoint_distance": "16384",
             "compaction_period": "1 s",
             "compaction_threshold": "1",
-            "compaction_target_size": "8192",
+            "compaction_target_size": "16384",
         }
     )