appear in saved wxme format files
also, improve the testing support for testing snip loading
(before this, the testing infrastructure could let one test
"leak" into another one in a way that could mask failures)
please include in release branch
Since that requires `racket/draw', which requires libcairo, which is
missing on some build machines. It might be needed in the future, but
for now it's better to make it easier to add build machines.