Configuration unions

We should support a `union` operator in the configuration file:

``` yaml
preset:
browser:
union:
- platforms: [chrome, content-shell]
- include_tags: [browser]
```

It would run multiple more-or-less separate test runs with different configuration, but within the same process and using the standard test runner parallelization and unified reporter.

We'd have to define what configuration would be allowed in a union. All test configuration would be allowed, but some runner configuration (such as `concurrency` or `reporter`) wouldn't make sense. We'd probably want to define a separate category, "suite configuration", that includes the subset of what is now runner configuration that involves selecting or configuring entire test suites.

The issue names no files, tests, or entry points. Start by tracing configuration-file handling and the standard test runner, then define which suite settings unions accept and verify that separate configurations run in one process with shared parallelization and reporting.

Written by the indexing model from the issue text.