Inline some zone interactions to a single function (#1481)

Combine the behavior of `Invoker.unclosable`, and `errorsDontStopTest`
into a single method since they were previously used in two places to
wrap the same intent.

Add a `runTearDowns` method on `Invoker`. Use it from `Declarer` where
it is running `tearDownAll` callbacks, and from `Invoker` where it is
running a test's `tearDown` callbacks. This is the only place (outside of
tests internal to this repo) that the `waitForOutstandignCallbacks` zone
will be nested and the only place where an `Invoker` is not closable. The
implementation is intended to provide the following properties:

- The outer "test", which may be either a `test` or a `tearDownAll`,
  will not be considered complete until all outstanding callbacks from
  all tear down callbacks are also complete. This is the reason for a
  call to `addOutstandingCallback` in `runTearDowns` which doesn't get
  removed until all outstanding callbacks from the tear down are also
- Tear downs do not need to wait for all outstanding callbacks from
  previous tear downs before they can start. They do wait for the
  `Future` returned from prior tear downs. This is the reason that the
  loop waits for the `Completer` which is completed following the tear
  down callback, and the remainder of the `waitForOutstandingCallbacks`
  work is `unawaited`.
- If an error is thrown within a tear down callback, only that tear
  down's outstanding callbacks will be ignored - other tear down
  callbacks will continue to hold the test open as normal and the test
  will not be considered complete the way it would be if the error had come
  from the test body. This is the reason for the zone nesting, an error
  will make it seem as though outstanding callbacks completed in only
  the zone where the error is raised.
- Give a verbose name and a longer doc comment to the `closable`
  fields. Documents clearly the single intended purpose it has today, and
  should prevent it from being accidentally given an extra purpose without
  updating the doc.
- Remove the future that never completes part. As far as I can tell, it
  isn't an exercised code path today, and if a usage somehow cropped up
  it's very unlikely that a non-completing future would be the right

Make `waitForOutstandingCallbacks` private and rewrite the tests to call
4 files changed