EuiComboBox keyboard navigation: tab exit

[thompsongl]

Summary

Fixes #1451 by adding a bit more logic to determine whether to allow or disallow tab from exiting EuiComboBox focus.
Note that the bug only impacted the single select variation of the combo box.

Testing/reproduction can be done in the EUI docs site (Combo Box > Single Selection):

• Tab into the combo box
• Use arrow keys to select an option
• Hit enter
• Hit tab; caret should have focus
• Hit tab again; focus should move to the next relevant element

Previously, the last step would never move focus off the caret

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs
- [ ] Documentation examples were added

• A changelog entry exists and is marked appropriately

- [ ] This was checked for breaking changes and labeled appropriately
- [ ] Jest tests were updated or added to match the most common scenarios

• This was checked against keyboard-only and screenreader scenarios

- [ ] This required updates to Framer X components

[snide]

This works!