tests: wait for flush lsn in test_branch_creation_before_gc (#12527)

## Problem
Test `test_branch_creation_before_gc` is flaky in the internal repo.
Pageserver sometimes lags behind write LSN. When we call GC it might not
reach the LSN we try to create the branch at yet.

## Summary of changes
- Wait till flush lsn on pageserver reaches the latest LSN before
calling GC.

test_runner/regress/test_branch_and_gc.py

@@ -7,6 +7,7 @@ from typing import TYPE_CHECKING
 import pytest
 from fixtures.common_types import Lsn, TimelineId
 from fixtures.log_helper import log
+from fixtures.neon_fixtures import wait_for_last_flush_lsn
 from fixtures.pageserver.http import TimelineCreate406
 from fixtures.utils import query_scalar, skip_in_debug_build
 
@@ -162,6 +163,9 @@ def test_branch_creation_before_gc(neon_simple_env: NeonEnv):
     )
     lsn = Lsn(res[2][0][0])
 
+    # Wait for all WAL to reach the pageserver, so GC cutoff LSN is greater than `lsn`.
+    wait_for_last_flush_lsn(env, endpoint0, tenant, b0)
+
     # Use `failpoint=sleep` and `threading` to make the GC iteration triggers *before* the
     # branch creation task but the individual timeline GC iteration happens *after*
     # the branch creation task.