Make it possible to use decorator syntax with tests

It would be nice to support decorator syntax within tests. This would make it possible to use a QuickCheck tool such as [Paycheck](https://github.com/markchadwick/paycheck). This implies the tool should be able to inject new arguments to tests automatically somehow.

The issue names no files or tests. Start by locating the test runner's test-discovery and invocation entry points, then review how decorator syntax and Paycheck-style argument generation could be represented. Done means decorated tests run successfully with automatically injected arguments, with coverage for that behavior.

Written by the indexing model from the issue text.