Session scoped fixtures

[obmarg]

Now that there is a global fixture module registry, it should be possible to implement session scoped fixtures. We can store details about these in the registry and try to create them as close to the test start time as we can. A few points:

• I'm not sure how to determine the test start time. We could probably do this as the first test setup we run into, but this might have some problems. Async tests might also be running, and also some non-fixture using tests might have run beforehand. Not sure if this is a problem, but things that will need to be thought about.
• Session scoped fixtures may need to be defined in fixture modules rather than globally. Though this probably depends on whether ExUnit compiles all tests before starting running tests or not. Need to look into that.

[obmarg]

Thinking about this some more: I might not need to precisely determine test start time. It'd probably be OK to only create session scoped fixtures when we first encounter a test that depends on them. We'd probably need a small amount of co-ordination/locking to ensure that 2 async test modules couldn't both create session fixtures at the same time, but that should be relatively trivial.

Presumably we'd need a way to tear down session scoped fixtures. Not so sure how we'd do this, but can look into it.