Show the gameplay test run button at the center of the toolbar

[Bouh]

Description

montage-mtipq0eg.mp4

The button used to run a gameplay test was displayed at the right of the toolbar, next to the properties panel icon, while every other editor shows its preview button at the center. It is now displayed in its own centered toolbar group, taking the place of the preview button.

The properties panel had a second "Run the test" button, which is removed: a single button remains, always visible even when the panel is closed. The panel keeps the progress indicator shown while a test is running.

Changes

• GameplayTestEditorToolbar: the run/stop button moves to its own centered ToolbarGroup, the properties panel icon stays in the trailing group.
• MainFrame/Toolbar: the empty placeholder ToolbarGroup is not rendered when there is no preview button, so the editor toolbar can provide the centered group itself. This only concerns the gameplay test editor, the only one setting showPreviewAndShareButtons to false while showing the project buttons.
• GameplayTestProperties: the duplicated run/stop buttons are removed, along with the now unused onRunTest and onStopTest props (and their plumbing in GameplayTestEditor, GameplayTestEditorContainer and the story).

Checklist

• Prettier and ESLint pass on the changed files.