Add a test/script key to simplify specifying test scripts [testing]

This MR adds a new supported test/script(s) key for the package YAML files, to simplify specifying test scripts, in a similar way to the test/script key used by conda-build.

This is inspired by an idea to run all tests in their own unique temporary directory, but I haven't quite figured out how to get the individual test script into that directory yet, so I'll do that later.