fix a test for `raco exe`

The test failed for certain installation configurations due to a
problem with the test.
raco exe: document command-line flags
sync: accept 0 arguments

As suggested by Jonathan Schuster.

Note that the `choice-evt` constructor already accepted 0 arguments.
racket/draw: fix `get-argb-pixels` for unscaled and just-alpha

Closes PR 14653

Merge to v6.1