Add CPU browser tests to `vello_sparse_tests`

[ajakubowicz-canva]

This is a followup to the WebGL browser tests added in #1020 , and prompted by comment #1020 (comment)

Current state:

• sparse_strips CPU renderer tests do not run in the browser.

Completion criteria:

• sparse_strips CPU renderer tests run in the browser.

[DJMcNab]

Was this impacted by #1065?

[ajakubowicz-canva]

Thank you! This is done and closed by #1065