add 'permutations' to 'tests' package.py attrib

We've hit a case where we'd like to have a test run against multiple python versions, but the package doesn't have (or need) variants. The only way to do this currently is to introduce redundancy and define multiple tests, with differing py version in each test's `requires` param.

We could instead add a new `permutations` field, like so:

```
tests = {
"unit": {
"command": "...",
"permutations": [
{ "py2": { "requires": ["python-2"]} },
{ "py3": { "requires": ["python-3"]} },
]
}
}
```

This would expand the list of tests out to two in this case - `unit.py2` and `unit.py3`. The dict in each permutation is usedto update the test, so in this case the test "requires" field is different in each test.

Start by locating the package.py handling for the tests attribute and the code that expands test definitions. Trace how requires is applied to individual tests, then determine how named permutations should produce tests such as unit.py2 and unit.py3. Done means the permutations example expands into separate tests with the per-permutation requires values.

Written by the indexing model from the issue text.