Test: API - support group ordering for menu contributions

[Tyriar]

Test for #9153:

• any os - @chrisdias
• macOS - @weinand

When using the menuitems contribution point, you can control the order of menu items and of menu item groups. Verify that:

• groups are sorted lexicographically - except for the navigation group which comes first always
• menu items are sorted lexicographically when they just specify a group. When the group matches a pattern like some_group@10 the number used to promote/demote the item.

[weinand]

@jrieken I suggest to mention the predefined non-obvious group names in the doc, e.g. the name of the "Cut/Copy/Paste" group.

[jrieken]

Agree - we should document the groups we define by default