Use (racket-proc) instead of hardcoded "*racket*"
Revise options for running.

- Clarify that the DrRacket style "Run" is using `enter!`.

- Change the Run Test command to use a shell buffer to execute `raco
  test`, rather than doing a `(system*)` in the Racket inferior
  process. This is more reliable, e.g. works with `#lang racket/base`.

- Add a Run command that works like Run Test, i.e. invokes `racket
  <file>` using a Shell buffer. (Probably this will need an option for
  racket command-line flags, but for now it uses none.)
Improve some command names.