wacom: Update "Test your settings" button sensitivity on device availability

The button/popover are meaningless if there's no device to test with. Set
it inactive (so the popover hides if visible) and set insensitive if no
devices are found.
11 jobs for <span class="ref-name">wip/carlosg/wacom-test-button-sensitivity</span> in 4 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
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  Test
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  Review
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review
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