test: More precise click on CPU shares number input

Enabling the disabled element only works when clicking on the [ - ] or
[ + ] buttons, not the number itself. With Firefox the test will soon
click into the center of the element, which is the number. Make the
selector more specific to ensure it goes to the [ - ] button.

Also circumvent the "element must be not enabled" restriction by
directly using `Browser.mouse()`. This has always been a race condition,
as we expect the number input to be disabled by default.

test/check-application

@@ -1865,7 +1865,7 @@ class TestApplication(testlib.MachineCase):
         if auth:
             # Check that the checkbox is enabled when clicked on the field
             b.wait_visible("#run-image-dialog-cpu-priority-checkbox:not(:checked)")
-            b.click('#run-image-cpu-priority')
+            b.mouse('#run-image-cpu-priority button[aria-label="Decrease CPU shares"]', "click")
             b.wait_visible("#run-image-dialog-cpu-priority-checkbox:checked")
             b.set_checked("#run-image-dialog-cpu-priority-checkbox", False)