add a test for WebGL 2.0 Compute

The test is expected to work on Windows and Linux with
EMTEST_BROWSER="chrome --use-gl=angle --use-angle=gl --use-cmd-decoder=passthrough --enable-webgl2-compute-context".

Sadly, the Mesa software renderer is too slow and lacks certain OpenGL
extensions for WebGL 2.0 Compute to work, so the test is not enabled in
CI.

Author: hujiajie