The YKI (Yleinen kielitutkinto) has four parts: listening, speaking, reading and writing. The listening part is at normal speed, in the Finnish people actually speak, which isn't what most courses teach. If you've only studied kirjakieli, this is the part that can be hard.
SaunaSpeak is not a YKI course. It doesn't help with the reading and writing parts, it doesn't mark practice tests, and it can't tell you if you're ready for the test. What it can help with is the listening part: understanding everyday spoken Finnish at normal speed.
Usually the problem isn't new words. It's that words you already know sound different. You can read "minä olen" on paper, but spoken it's "mä oon", and if you've never seen that form, it's hard to catch.
Test dates, levels, fees and the citizenship requirements are on the official site at ykitesti.solki.jyu.fi.