new test for LFC stats in explain (#6968)

## Problem

PR https://github.com/neondatabase/neon/pull/6851 implemented new output
in PostgreSQL explain.
this is a test case for the new function.

## Summary of changes

## Checklist before requesting a review

- [x] I have performed a self-review of my code.
- [x] If it is a core feature, I have added thorough tests.
- [no ] Do we need to implement analytics? if so did you add the
relevant metrics to the dashboard?
- [no] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

test_runner/regress/test_explain_with_lfc_stats.py

@@ -0,0 +1,84 @@
+from pathlib import Path
+
+from fixtures.log_helper import log
+from fixtures.neon_fixtures import NeonEnv
+
+
+def test_explain_with_lfc_stats(neon_simple_env: NeonEnv):
+    env = neon_simple_env
+
+    cache_dir = Path(env.repo_dir) / "file_cache"
+    cache_dir.mkdir(exist_ok=True)
+
+    branchname = "test_explain_with_lfc_stats"
+    env.neon_cli.create_branch(branchname, "empty")
+    log.info(f"Creating endopint with 1MB shared_buffers and 64 MB LFC for branch {branchname}")
+    endpoint = env.endpoints.create_start(
+        branchname,
+        config_lines=[
+            "shared_buffers='1MB'",
+            f"neon.file_cache_path='{cache_dir}/file.cache'",
+            "neon.max_file_cache_size='128MB'",
+            "neon.file_cache_size_limit='64MB'",
+        ],
+    )
+
+    cur = endpoint.connect().cursor()
+
+    log.info(f"preparing some data in {endpoint.connstr()}")
+
+    ddl = """
+CREATE TABLE pgbench_accounts (
+    aid bigint NOT NULL,
+    bid integer,
+    abalance integer,
+    filler character(84),
+    -- more web-app like columns
+    text_column_plain TEXT  DEFAULT repeat('NeonIsCool', 5),
+    jsonb_column_extended JSONB  DEFAULT ('{ "tell everyone": [' || repeat('{"Neon": "IsCool"},',9) || ' {"Neon": "IsCool"}]}')::jsonb
+)
+WITH (fillfactor='100');
+"""
+
+    cur.execute(ddl)
+    cur.execute(
+        "insert into pgbench_accounts(aid,bid,abalance,filler) select aid, (aid - 1) / 100000 + 1, 0, '' from generate_series(1, 100000) as aid;"
+    )
+
+    log.info(f"warming up caches with sequential scan in {endpoint.connstr()}")
+    cur.execute("SELECT * FROM pgbench_accounts WHERE abalance > 0")
+
+    log.info("running explain analyze without LFC values to verify they do not show up in the plan")
+    cur.execute("EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM pgbench_accounts WHERE abalance > 0")
+    rows = cur.fetchall()
+    plan = "\n".join(r[0] for r in rows)
+    log.debug(plan)
+    assert "Seq Scan on pgbench_accounts" in plan
+    assert "Buffers: shared hit" in plan
+    assert "File cache: hits=" not in plan
+    log.info("running explain analyze WITH LFC values to verify they do now show up")
+    cur.execute(
+        "EXPLAIN (ANALYZE, BUFFERS,FILECACHE) SELECT * FROM pgbench_accounts WHERE abalance > 0"
+    )
+    rows = cur.fetchall()
+    plan = "\n".join(r[0] for r in rows)
+    log.debug(plan)
+    assert "Seq Scan on pgbench_accounts" in plan
+    assert "Buffers: shared hit" in plan
+    assert "File cache: hits=" in plan
+    log.info("running explain analyze WITH LFC values to verify json output")
+    cur.execute(
+        "EXPLAIN (ANALYZE, BUFFERS,FILECACHE, FORMAT JSON) SELECT * FROM pgbench_accounts WHERE abalance > 0"
+    )
+    jsonplan = cur.fetchall()[0][0]
+    log.debug(jsonplan)
+    # Directly access the 'Plan' part of the first element of the JSON array
+    plan_details = jsonplan[0]["Plan"]
+
+    # Extract "File Cache Hits" and "File Cache Misses"
+    file_cache_hits = plan_details.get("File Cache Hits")
+    file_cache_misses = plan_details.get("File Cache Misses")
+
+    # Now you can assert the values
+    assert file_cache_hits >= 5000, f"Expected File Cache Hits to be > 5000, got {file_cache_hits}"
+    assert file_cache_misses == 0, f"Expected File Cache Misses to be 0, got {file_cache_misses}"