Use Execution to run flows in REPL

- [x] Use Execution (from #915) to run REPL flows (rather than creating fake "Job"s)
- [x] Add unit tests for functionality added in #918 using Executions.
I wanted to have a test that ran the snapshot in isolation, but there's not really a good way to introspect it without running another flow to get the contents, so I just wanted a simple check that it actually created the TypedPipe it was supposed to.