Calendar events are written as soon as a booking is created, and there's no per-booking switch to suppress that. So the way to test safely is to choose a subject who has no calendar connected.
Two safe ways to test
Use an artist with no calendar linked.
Go to Settings > Calendar & Templates > Calendars.
Search for the artist you're thinking of using.
Confirm no calendar is linked to them.
Create your test booking with that artist attached. With nothing linked, nothing is written anywhere.
Create a temporary test artist.
Add a new artist to your roster with an obvious name like "Test Artist".
Don't link a calendar to them and don't add members.
Create your test booking against that artist.
Delete the test booking, then the test artist, as soon as you're finished.
Clean up before you generate anything
Delete the test while it's still just a booking. Once a document or invoice exists against it, the artist can't be removed from the event any more, and you're left with a test record you can't fully tidy away.
So: create it, check what you needed to check, delete it. Don't send a contract, issue an invoice, or generate documents from a test booking.
If that doesn't work
If you need to test something that only happens on a real artist — a specific calendar behaviour, or how an event looks once it lands — a workable approach is to link a scratch calendar you own to the test artist rather than the artist's real one. You see the result and nobody else does.
Good to know
Unlinking a live artist's calendar to run a test isn't a good substitute. Bookings created while it's unlinked won't appear once you reconnect it, so you can end up with gaps on a real artist's calendar that nobody notices for weeks.
If you only want to change what appears on calendar events rather than stop them entirely, the options for that live at the bottom of Settings > Presets > Events.