Fix load suite timeout weirdness

Closes #390

This can never fire before the load suite timeout. Now that that
timeout actually works, it's redundant.

See #390

* Fix fuzzy arrow warnings since they will become errors before too long.

* Keep type on _onMessage() parameter.

We weren't completing the _suiteAndZone completer, which meant that
errors (such as timeouts) were reported but didn't actually stop the
suite from loading.

See #390

Make addTearDown() play nice with setUpAll()

addTearDown() now adds a tearDownAll() rather than adding a plain
tearDown() when called from within setUpAll() (or tearDownAll()). This
is technically a behavioral change, but the old behavior was so
useless I doubt anyone was relying on it.

Closes #712

This allows us to avoid issues like those described in #713 where we
want users to be able to share errors between different logical tests.

Use Stdout.terminalColumns for line length

Fall back on a 200-character default, which is less likely to obscure
test names when printing to a non-terminal destination.

Closes #86 

This makes it possible to use the same spawnHybridUri() call from
multiple test files that are in different places in the filesystem.

Add the ability to define and override test platforms

See #99
Closes #391

See #391

This avoids an annoying "failed to load" message in the console.

Closes #637

See #391

Add an override_platforms config field

See #391

Add a define_platforms configuration field

See #391

See #391

This doesn't really do anything yet, since no platforms currently
support customization.

See #391

This makes it easier to add platforms that extend existing platforms
without running into serialization headaches. It also cleans up the
Declarer API and means we're sending less redundant data between
isolates.

Now that the Loader is the canonical place for determining which
TestPlatforms are valid, we no longer need a separate mechanism for
registering platforms. Adding a plugin to the loader is sufficient.

See #391

This ensures that the remote listener has access to any platforms that
are dynamically loaded in the test runner, so they can be used in
platform selectors.

See #99
See #391

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

This will make it easier to define custom test platforms (both via #391
and #99) in the future. Because those platforms will be loaded based on
the configuration, requiring knowledge of them to parse the user's
platform arguments would produce an unresolvable circular dependency.

This makes it possible to commit reviewed code to a branch when working
on a larger change that's composed of multiple pull requests.

Closes #688