exclude separate perf tests from bench step (#11084)

## Problem

Our benchmarking workflow has a job step `bench`which runs all tests in
test_runner/performance/* except those that we want to run separately.
We recently added two test cases to that testcase directory that we want
to run separately but forgot to ignore them during the bench step. This
is now causing
[failures](https://github.com/neondatabase/neon/actions/runs/13667689340/job/38212087331#step:7:392).

## Summary of changes

Ignore the separately run tests in the bench step.

.github/workflows/benchmarking.yml

@@ -141,6 +141,8 @@ jobs:
           --ignore test_runner/performance/test_physical_replication.py
           --ignore test_runner/performance/test_perf_ingest_using_pgcopydb.py
           --ignore test_runner/performance/test_cumulative_statistics_persistence.py
+          --ignore test_runner/performance/test_perf_many_relations.py
+          --ignore test_runner/performance/test_perf_oltp_large_tenant.py
       env:
         BENCHMARK_CONNSTR: ${{ steps.create-neon-project.outputs.dsn }}
         VIP_VAP_ACCESS_TOKEN: "${{ secrets.VIP_VAP_ACCESS_TOKEN }}"