Add release checklist test: "GRacket Tests"

[jackfirth]

Owned by Matthew Flatt. Only covers the racket -l tests/gracket/test command, not the gracket -z and gracket-text commands. The latter two are expected to be run on Windows and Mac; supporting those with our current docker setup is a much larger project.

Note that the gracket tests depend on the directory they're run in, see racket/gui#82.

[samth]

I think we can just do this, by taking advantage of the (probably bad) feature of raco test that changes directory to the file being run.

That is, raco test -l tests/gracket/test should just do the right thing.

[jackfirth]

Sneaky sneaky