We want users to be able to dynamically define new platforms, which
means we need infrastructure in place for piping those platforms to
places that previously assumed TestPlatform.all was a full list of
available platforms. PlatformSelector is the trickiest example, since
it's parsed in a number of different places and needs to provide useful
feedback to users when they use an undefined platform.

This splits parsing and platform validation into two separate steps.
Validation will be done immediately after parsing when the selectors
come from top-level annotations or parameters passed to test() or
group(), but selectors defined in configuration files are now parsed
only after all configuration is parsed. This will allow new platforms to
be defined *and* referenced in configuration files.

See #99
See #391