Playing around with end-to-end testing

[dtinth]

This PR:

• Implements a TEST MODE which allows the game to be paused at a prespecified point. This is to facilitate synchronization with test executor.
• Create a simple test that runs the game and plays through a song and verifies the score.

Tasks:

• Prototype an E2E test.

• Set up CI to run it (maybe using CircleCI workflow to copy the generated dist).

[codecov-io]

Codecov Report

Merging #503 into master will decrease coverage by 0.27%.
The diff coverage is 9.52%.

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
- Coverage   83.52%   83.25%   -0.28%     
==========================================
  Files         171      172       +1     
  Lines        5403     5423      +20     
  Branches        1        1              
==========================================
+ Hits         4513     4515       +2     
- Misses        890      908      +18

Impacted Files	Coverage Δ
src/online/scoreboard-system/OnlineService.js	1.53% <0%> (-0.05%)	⬇️
src/devtools/BemuseTestMode.js	11.11% <11.11%> (ø)

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