Make shell unit testing harness work again
Adapt graphics unit tests to serialization format changes
Add getter for Stage#alpha

Stage overrides tha `alpha` setter. Unfortunately, not specifying a getter in the same child class that overrides a setter leaves the getter undefined ...
Fix Stage unit tests

Some fixes for correctness issues in the tests themselves, plus changed all `check` calls to equivalent `eq` calls.
Turn failing bitmap unit test into warning

All active tests should be green.
Fix shell unit testing harness

Also adds the ability to run test file collection files, used in the next patch.
Run shell unit tests after all relevant build targets

This introduces a unit tests collection file `shell-tests.js` and runs it each time one of the `swf`, `flash`, or `player` build targets is invoked.

Don't regress these tests from hereon out!