[autotests] Fix decoration input test

Since d51b8dc09393 the test fails on CI, apparently because we test with
Breeze default settings, which is no borders now and we can move a bit into
the window geometry and still be on the border if borders exist, otherwise
not.

For now fix it by checking if there are borders or not and then test
accordingly. But long-term we should test both cases and besides not rely on
external decorations for our test, instead use a fake specific for our
integration testing.