fix(testing): Use 1 MB shared_buffers even with LFC (#9969)

## Problem

After enabling LFC in tests and lowering `shared_buffers` we started
having more problems with `test_pg_regress`.

## Summary of changes

Set `shared_buffers` to 1MB to both exercise getPage requests/LFC, and
still have enough room for Postgres to operate. Everything smaller might
be not enough for Postgres under load, and can cause errors like 'no
unpinned buffers available'.

See Konstantin's comment [1] as well.

Fixes #9956

[1]:
#9956 (comment)

control_plane/src/endpoint.rs

@@ -310,6 +310,10 @@ impl Endpoint {
         conf.append("wal_log_hints", "off");
         conf.append("max_replication_slots", "10");
         conf.append("hot_standby", "on");
+        // Set to 1MB to both exercise getPage requests/LFC, and still have enough room for
+        // Postgres to operate. Everything smaller might be not enough for Postgres under load,
+        // and can cause errors like 'no unpinned buffers available', see
+        // <https://github.com/neondatabase/neon/issues/9956>
         conf.append("shared_buffers", "1MB");
         conf.append("fsync", "off");
         conf.append("max_connections", "100");

test_runner/fixtures/neon_fixtures.py

@@ -3801,13 +3801,11 @@ def create(
                     assert size_to_bytes(size) >= size_to_bytes(
                         "1MB"
                     ), "LFC size cannot be set less than 1MB"
-            # shared_buffers = 512kB to make postgres use LFC intensively
-            # neon.max_file_cache_size and neon.file_cache size limit are
-            # set to 1MB because small LFC is better for testing (helps to find more problems)
             lfc_path_escaped = str(lfc_path).replace("'", "''")
             config_lines = [
-                "shared_buffers = 512kB",
                 f"neon.file_cache_path = '{lfc_path_escaped}'",
+                # neon.max_file_cache_size and neon.file_cache size limits are
+                # set to 1MB because small LFC is better for testing (helps to find more problems)
                 "neon.max_file_cache_size = 1MB",
                 "neon.file_cache_size_limit = 1MB",
             ] + config_lines