Previously, the `obj` of a `TestCaseFunction` (the unittest plugin item
type) was the unbound method. This is unlike regular `Class` where the
`obj` is a bound method to a fresh instance.
This difference necessitated several special cases in in places outside
of the unittest plugin, such as `FixtureDef` and `FixtureRequest`, and
made things a bit harder to understand.
Instead, match how the python plugin does it, including collecting
fixtures from a fresh instance.
The downside is that now this instance for fixture-collection is kept
around in memory, but it's the same as `Class` so nothing new. Users
should only initialize stuff in `setUp`/`setUpClass` and similar
methods, and not in `__init__` which is generally off-limits in
`TestCase` subclasses.
I am not sure why there was a difference in the first place, though I
will say the previous unittest approach is probably the preferable one,
but first let's get consistency.
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