Make Scratch 3 project timer more compatible with Scratch 2 currentMSecs usage

These compatibility changes:
 - Use runtime.currentMSecs for the Clock timer "now" value
 - Start executing hats before other threads change values
 - Update test and fixtures to work with earlier hat execution
 - Add test for hat execution block order

src/engine/sequencer.js

@@ -138,7 +138,6 @@ class Sequencer {
                     activeThread.status === Thread.STATUS_DONE) {
                     // Finished with this thread.
                     stoppedThread = true;
-                    this.runtime.updateCurrentMSecs();
                 }
             }
             // We successfully ticked once. Prevents running STATUS_YIELD_TICK