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This change:
1. Adds the earcons lesson to the interactive tutorial. Functionality
for this is included in buildEarconLesson() in i_tutorial.js
2. Adds a test for the lesson.
3. To reduce flakiness, we needed to change how we wait for
the tutorial. Now, we wait for all lesson content to finish loading
before we perform commands and make assertions.

focused. This change also includes an automated test.

Test: Manually verify on-device that earcons are played when text is
Change-Id: I89d2f3c050a74b8d35df47091d88ba443105dadb
Fixed: 1124078
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426968
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813035}