use a prod-like shared_buffers size for some perf unit tests (#11373)

## Problem

In Neon DBaaS we adjust the shared_buffers to the size of the compute,
or better described we adjust the max number of connections to the
compute size and we adjust the shared_buffers size to the number of max
connections according to about the following sizes
`2 CU: 225mb; 4 CU: 450mb; 8 CU: 900mb`

[see](877e33b428/goapp/controlplane/internal/pkg/compute/computespec/pg_settings.go (L405))

## Summary of changes

We should run perf unit tests with settings that is realistic for a
paying customer and select 8 CU as the reference for those tests.

test_runner/performance/test_parallel_copy.py

@@ -7,6 +7,8 @@ from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from fixtures.neon_fixtures import Endpoint, NeonEnv
 
+from fixtures.utils import shared_buffers_for_max_cu
+
 
 async def repeat_bytes(buf, repetitions: int):
     for _ in range(repetitions):
@@ -45,7 +47,10 @@ async def parallel_load_same_table(endpoint: Endpoint, n_parallel: int):
 # Load data into one table with COPY TO from 5 parallel connections
 def test_parallel_copy(neon_simple_env: NeonEnv, n_parallel=5):
     env = neon_simple_env
-    endpoint = env.endpoints.create_start("main")
+    # use shared_buffers size like in production for 8 CU compute
+    endpoint = env.endpoints.create_start(
+        "main", config_lines=[f"shared_buffers={shared_buffers_for_max_cu(8.0)}"]
+    )
 
     # Create test table
     conn = endpoint.connect()