Add missing test cases newCursorForMoveRightWord()

[kyu08]

PR Description

I noticed that pkg/gocui/text_area.go#L357-L359 is not covered by unit test when I was implementing #5804.

So I added insufficient test cases.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc