Use mixed order insertions

Use mixed order insertions

Currently we have a unit test to test that signers are sorted by
ordering. We call `add_external` to add them but currently we add them
in the same order we expect them to be in. This means if the
implementation happens to insert them simply in the order they are
added (i.e. insert to end of list) then this test will still pass.

Insert in a mixed order, including one lower followed by one higher -
this ensures we are not inserting at the front or at the back but are
actually sorting based on the `SignerOrdering`.