fix(pagesever): stablize gc-compaction tests (#10621)

## Problem

Hopefully this can resolve
https://github.com/neondatabase/neon/issues/10517. The reason why the
test is flaky is that after restart the compute node might write some
data so that the pageserver flush some layers, and in the end, causing
L0 compaction to run, and we cannot get the test scenario as we want.

## Summary of changes

Ensure all L0 layers are compacted before starting the test.

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/regress/test_compaction.py

@@ -250,6 +250,9 @@ def test_pageserver_gc_compaction_idempotent(
     workload.churn_rows(row_count, env.pageserver.id)
     # compact 3 times if mode is before_restart
     n_compactions = 3 if compaction_mode == "before_restart" else 1
+    ps_http.timeline_compact(
+        tenant_id, timeline_id, force_l0_compaction=True, wait_until_uploaded=True
+    )
     for _ in range(n_compactions):
         # Force refresh gc info to have gc_cutoff generated
         ps_http.timeline_gc(tenant_id, timeline_id, None)