tests: Fix edje decorators test
Commit dd584b9f79ec41b53ca620ae534fcf84f12e30a9 fixed bad behaviour in edje entry.
Our test was picking up the "changed" and "cursor,changed" signals emitted by edje entry
when its text was set to "" on creation. Raster changed this behaviour, removing the
emissions, so let's do the same in our test and add some future proofing as well.