Switching to "random backdrop" should always select a different backdrop (Closes #521)

[aspizu]

Resolves: #521

[adazem009]

You forgot to update the test at line 2500. You'll have to set the costume index to 0 before the EXPECT_CALL though, because the default index is -1 (when there are no costumes).

stage.setCostumeIndex(0);