Update tests/import-debian-package/01-basic.branches to reflect new behavior

A new temporary branch (proposed-package/apertis/v2020) is created
to generate debian/apertis/copyright.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

tests/import-debian-package/01-basic.branches

-* apertis/v2020
+  apertis/v2020
   apertis/v2021
   debian/buster
   pristine-lfs
+* proposed-package/apertis/v2020
   upstream/buster