To racket-tests.el add install/enable company-mode

[greghendershott]

• As mentioned on Using racket-mode to edit source in org-mode document causing 100% cpu #348, I like completion... just not auto-completion. As a result, I don't enable company-mode in my own real work. Many racket-mode features get "dog-fooded" by me nearly daily. Unfortunately this doesn't include company-mode, which is justifiably popular. From time to time, bugs crop up, most recently REPL periodically freezes Emacs #344 and Using racket-mode to edit source in org-mode document causing 100% cpu #348.

• I do try to remember to test things manually with company-mode. But I don't always have time. Plus sometimes guess "oh this change probably won't require re-checking with company-mode" -- and we all know the quality of such guesses.

• I never commit directly to master. I always create topic branches, push them here, wait for CI to run. I set GitHub not to let me merge to master unless checks are all-green.

As a result, it would help to add something to racket-tests.el:

• package-install company-mode.
• enable it in a buffer.
• "type" some characters followed by RET.
• check that the expected full text was put into the buffer by company-mode -- and we didn't, say, hang.

I've been kind of swamped lately so adding the help-wanted label on the chance someone else might want to help add this?