Add integration test framework

[Celend]

No description provided.

[zshipko]

This is a great idea, but is quite a bit of work to implement across all the SDKs and PDKs. I have the start of something similar here: https://github.com/extism/pdk-test but without the configurable parts. Working on pdk-test really illuminated how complex testing a plugin like this would be, which is why it's stalled a little. If you want to start hacking on some of these ideas using pdk-test as a starting point that could be pretty interesting!

[Celend]

Yes, lot of works, I'll try to push it in the next few weeks, my project also need an integration test suite for multiple host languages.