Add decorators to control ordering

[ewencp]

For some tests it's useful to specify ordering -- it makes more sense to validate simple operations before more complex ones. It would be useful to have 2 types of annotations:

1. Specify dependencies which the test runner respects, i.e. @dependency(EverythingRunsTest) to annotate more complicated tests. This can also speed up failing test runs by immediately failing all dependent tests (or report them as SKIPPED).
2. Indicate a levels of testing, e.g. @smoke vs. @exhaustive. We'd need to think about the levels (or make them generic as @testlevel(1)) but the idea is that we could easily run a smaller set of tests that still give good coverage, but finish much faster.