plugins/testui: add keybinding to run all unit tests

src/plugins/testui/gtk/keybindings.json

+{ "trigger" : "<Control><Alt><Shift>u", "action" : "test-manager.test-all", "when" : "hasProject()", "phase" : "capture" },

src/plugins/testui/testui.gresource.xml

@@ -2,6 +2,7 @@
 <gresources>
   <gresource prefix="/plugins/testui">
     <file>testui.plugin</file>
+    <file>gtk/keybindings.json</file>
     <file preprocess="xml-stripblanks">gtk/menus.ui</file>
     <file preprocess="xml-stripblanks">gbp-testui-output-panel.ui</file>
     <file preprocess="xml-stripblanks">gbp-testui-panel.ui</file>