Fix test to be more robust with slow pageserver.

[hlinnaka]

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

[hlinnaka]

This fixes the flaky test discussed at #2063. I didn't comb through all the tests creating branches, though, so there might well be more instances of this. A more complete fix would be to always require an LSN when creating a branch, as discussed at #2063. So I don't consider this as a full fix for that issue.

[ololobus]

Didn't know that create branch already accepts LSN. This should works as far as I see, but that could be I don't know some details