raco test: create fresh user directory for each test in DrDr mode
tests: remove "gui" lock declarations

The fresh PLTUSERHOME created by `raco test --drdr` should avoid
the problems that made the lock necessary before.
raco test: add `++ignore-stderr <pattern>` option

I'm using this option to ignore "Xlib: extension \"RANDR\" missing"
warnings.
raco exe: adjust tests to avoid "stdout" in source directory
raco test: fix `--process` without `--fresh-user`
racket/sandbox: fix `with-deep-time-limit` test

The test has been failing consistently in DrDr, but I couldn't figure
out why until I learned that DrDr runs programs with an empty
stdin. It helps that `raco test --drdr` is now closer to DrDr.
disable embed-in-C test

Added to the release checklist, instead, since it requires a
`configure`-time flag.
fix parallel-build test
plot tests: write files to temp dir
raco test: continue when checking for submodules fails
slatex: generate files in temporary directory