Fixes cases where opening a convertible beatmap (so any osu! beatmap)
with the game-global ruleset being set to anything but osu! would result
in opening the editor gameplay test mode with the game-global ruleset
rather than the beatmap's.
Would leave the user potentially in a test mode that is in a weird
state (ie. if cinema mod was enabled). Eventually we'll add the ability
to choose mods for test play, but that will be done in a saner way.
Closes#15870.