Concurrent test runs where possible

[SamirTalwar]

In many cases, Smoke could potentially run multiple tests concurrently. Lots of the examples (e.g. the calculator) in the fixtures directory would work.

Because Smoke spawns programs that might do anything, I think this should be opt-in, behind a switch that sets the maximum number of concurrent tests (e.g. --concurrency=4).

Often, most of your tests can be run concurrently, but a few may not be able to. For example, tests that mutate files used by other tests are probably way out. This means that there needs to be a way to mark certain tests as serial, potentially with a YAML switch such as serial: true or exclusive: true.

I expect there's certain tests that should never be run concurrently. For example, if two tests refer to the same directory in a revert: declaration (or one refers to a descendant of another), those tests should never be run at the same time. However, they might be able to run in parallel with other tests. A simple implementation may not want to bother with this, though, and just force all tests with a revert: declaration to run serially.