Move aspell to CI tests

[coke]

The problem

Since this isn’t checked as part of CI but only author tests (xt/), we often miss typos until after we merge the PR.

Solution provided

Move to t/, might have to update CI to have the correct packages.

Closes #4788

[coke]

Updated docker test image to include aspell/aspell-en.

committed a fake word to a test file. Now getting:

A worker in a parallel iteration (hyper or race) initiated here:
  in block <unit> at t/23-aspell.rakutest line 71
Died at:
    A react block:
      in block  at t/23-aspell.rakutest line 105
    Died because of the exception:
        Malformed UTF-8 near bytes 65 e2 20
                                                   
t/23-aspell.rakutest ................ Dubious, test returned 255
All 2 subtests passed 

[coke]

This is very similar to #3339 and #4156

[coke]

Cleaned up and tested on the branch with a bad word that was correctly reported on. Thanks to @ShimmerFairy for the --encoding pointer!

Many force pushes to try things out, test a bad word, clean everything up; down to 2 clean commits for the merge.