tests: boxes: Add `SAVE_AS_DRAFT` keypress test for write box.

[mkp6781]

[mkp6781]

@zulipbot remove "PR awaiting update" @zulipbot add "PR needs review"

[mkp6781]

I have rebased this branch. Used Black to reformat the code so that it passes linting tests.

[zulipbot]

Heads up @mkp6781, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.

[neiljp]

Marking this as a completion candidate, but the underlying code has changed somewhat, so in addition to a rebase and improving the tests, this will need some adjustment to fit the new code - this will require digging for eg. renames and similar.

[criticic]

I'd like to work on this.