extra/runner/run: Don't run more tests if v4l2 capture device not found

This happens, for example, when the cleaners bump the power button on my
Hauppauge HDPVR that's running tests overnight.

Checking for "/dev/v" will match "/dev/video0" as well as
"/dev/v4l/by-id/..." (I use the latter in my source pipeline if the PC
has multiple devices connected, because the "by-id" path doesn't change
across PC reboots).
stbt.Display.teardown: Avoid additional exception after NoVideo timeout

If the NoVideo timeout happens after `restart_source` stopped the source
pipeline but before `start_source` started it again (there's a 5 second
gap), then the "NoVideo" exception would be followed by a second
exception message:

      File "/usr/libexec/stbt/stbt.py", line 920, in teardown
        self.source_pipeline.set_state(gst.STATE_NULL)
    AttributeError: 'NoneType' object has no attribute 'set_state'