Add support for setUpAll and tearDownAll.

CHANGELOG.md

 ## 0.12.5
 
+* Add `setUpAll()` and `tearDownAll()` methods that run callbacks before and
+  after all tests in a group or suite. **Note that these methods are for special
+  cases and should be avoided**—they make it very easy to accidentally introduce
+  dependencies between tests. Use `setUp()` and `tearDown()` instead if
+  possible.
+
 * Allow `setUp()` and `tearDown()` to be called multiple times within the same
   group.
 
 * When a `tearDown()` callback runs after a signal has been caught, it can now
   schedule out-of-band asynchronous callbacks normally rather than having them
   throw exceptions.
 
 ## 0.12.4+9
 
 * If a `tearDown()` callback throws an error, outer `tearDown()` callbacks are
@@ skipping 298 matching lines @@
 * **NEW!** `expectAsyncUntil` replaces the now deprecated `expectAsyncUntil0`,
     `expectAsyncUntil1` and `expectAsyncUntil2`
 * `TestCase`:
     * Removed properties: `setUp`, `tearDown`, `testFunction`
     * `enabled` is now get-only
     * Removed methods: `pass`, `fail`, `error`
 * `interactive_html_config.dart` has been removed.
 * `runTests`, `tearDown`, `setUp`, `test`, `group`, `solo_test`, and
   `solo_group` now throw a `StateError` if called while tests are running.
 * `rerunTests` has been removed.