Fix test to be more robust with slow pageserver.

If the WAL arrives at the pageserver slowly, it's possible that the
branch is created before all the data on the parent branch have
arrived. That results in a failure:

    test_runner/batch_others/test_tenant_relocation.py:259: in test_tenant_relocation
        timeline_id_second, current_lsn_second = populate_branch(pg_second, create_table=False, expected_sum=1001000)
    test_runner/batch_others/test_tenant_relocation.py:133: in populate_branch
        assert cur.fetchone() == (expected_sum, )
    E   assert (500500,) == (1001000,)
    E     At index 0 diff: 500500 != 1001000
    E     Full diff:
    E     - (1001000,)
    E     + (500500,)

To fix, specify the LSN to branch at, so that the pageserver will wait
for it arrive.

See #2063

test_runner/batch_others/test_tenant_relocation.py

@@ -26,7 +26,7 @@
     wait_for_upload,
     wait_until,
 )
-from fixtures.utils import lsn_from_hex, subprocess_capture
+from fixtures.utils import lsn_from_hex, lsn_to_hex, subprocess_capture
 
 
 def assert_abs_margin_ratio(a: float, b: float, margin_ratio: float):
@@ -268,6 +268,7 @@ def test_tenant_relocation(neon_env_builder: NeonEnvBuilder,
     env.neon_cli.create_branch(
         new_branch_name="test_tenant_relocation_second",
         ancestor_branch_name="test_tenant_relocation_main",
+        ancestor_start_lsn=lsn_to_hex(current_lsn_main),
         tenant_id=tenant_id,
     )
     pg_second = env.postgres.create_start(branch_name='test_tenant_relocation_second',