switch `gracket`-based DrDr tests to `raco test -m --fresh-user`

Various GUI (especially DrRacket) tests have used `(gracket *)` as
a signal that the tests must be serialized, but those tests can run
concurrently if run with a different "home" directory (as determined
by the `PLTUSERHOME` environment variable).

Change those test to use `raco test -m --fresh-user`, instead,
which runs the test with a fresh home directory.
meta/tests/unix-installer: disable for DrDr

The test is meant to be run manually at release time.
scribble: recognize 'exact-chars even when a style name is #f

Patch from Tero Hasu.