tests: Keep proc instance for test_base and test_osqueryd

[theopolis]

This is required for #6334.

I am unsure why the "runner"'s proc member was removed at the end of a successful run()?

But without this instance we cannot inspect the daemon's pid. In the follow up PR we confirm the daemon exits correctly.

[Smjert]

Ah good catch!
The problem was that the base framework doesn't always correctly cleanup resources when an operation has finished and this either keeps processes running or make the test fail in other parts.

I will eventually get to it again, but that framework needs a refactor.

EDIT:
Checking again, the intention was the above and to save the pid for later inspection, although what I missed at the time is that pid is a property, not a simple class member variable, that has to access the proc member.
I think it's fine to bring it back as it was, although as I was saying this needs fixing eventually.