tests: Test auto-alias generation on base tree, not overlay

The current testcases for the -A "auto alias generation" option operate on
a "plugin" tree.  Although not technically wrong, this is an odd approach,
since a plugin will almost certainly need the __symbols__ and/or __fixups__
syntax instead of aliases.  On the other hand -A may be useful simply for
generating aliases on a tree which is not using the overlay / plugin
mechanism at all.

Therefore change the tests to operate on a base tree example instead of a
plugin.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>