Adds interaction/render tests for the untested view layers: menu bar popup
open/click/keyboard-nav/escape, project explorer expand/open/view-switch,
scope widgets and keyable diamond rendering in windows, progress and file
dialog content rendering, and viewer transport buttons. gpui_widgets line
coverage rises from 75.4% to 89.4% (menu/mod 3.7%->71.9%, project_explorer
21.4%->91.3%, scopes/mod 0%->96.7%, keyable 69%->98.4%); every module is now
>=70%. 115 widget tests pass.
Mike-Solar
a64234936c