Strict concurrency for SAL tests (#3205)

Motivation:

The syscall abstraction layer tests aren't sendable clean.

Modifications:

- Factor out the setup/teardown of SAL tests into a SALContext
- SALContext a method to run a SAL test with two closues: one on the
hooked event loop and another on the calling thread.
- The EL bound closure is provided with the EL and locked boxes for the
user ot kernel and kernel to user boxes.
- The closure on the calling thread is provided with an 'assertions'
object which provides a view over the hooked selector.
- Note that there are still sendable holes in this code.
- Refactored the tests to use these APIs.

Results:

Safer tests.

Co-authored-by: Cory Benfield <[email protected]>

Author: glbrntt