Update CodexTest to test valid_alpha.

Add a static function to CodexTest, which consolidates decoding,
comparing to an expected SkCodec::Result, and optionally comparing to a
digest.

Test decoding non-opaque to opaque (fails) and premul to unpremul/vice
versa (succeeds).

BUG=skia:3475

Review URL: https://codereview.chromium.org/1277253003
2 files changed