During the alpha we added hundreds of unit and integration tests. More are being added all the time. In the shift to beta we've really narrowed the scope of the changes we're making. When there were breaking changes they were fixed very quickly, often within hours. We'll continue to work on improving our release process during the beta and beyond.
The main strategy is (and always has been) to write tests for any changes that are made. Some aspects of the framework are harder to test (mostly the templating and routing related features) so it's taken longer to get coverage in these areas.
Beyond improving the test suites, there's work going on setup code coverage and automated cross-browser tests in all the modules.