Alter default IsolationMode to InstancePerTest instead of InstancePerSpec

[LeoColman]

I know we discussed this in 3.0, 4.0, 5.0 and now again 😂

I believe this is an opportunity to replace the default isolationMode from InstancePerSpec to InstancePerTest. It's more in-line with what new users expect when coming from JUnit (majority of kotest new users)

[Kantis]

I'll add a vote against this, because I feel switching defaults this far into the project would be a nuisance to existing users. I haven't really got the impression that anyone is bothered by this default. Maybe the votes on this discussion will prove me wrong though 👀

[LeoColman]

I'm pretty bothered by the default because I always forget about it and have to add InstancePerTest to all tests that depends on automatic cleanup.

Even after using Kotest since KotlinTest I still forget about it, that's why I feel this change deserves some love

[sksamuel]

In theory it might be better but I agree with @Kantis we're far too down the line to switch the default now.
Plus you can change it globally with config right.