samples: usb: audio: Fix building off sample with sanitycheck

The USB audio samples when added didn't have any tests: sections in the
sample.yaml so they would never get build on any platform as part of
sanitycheck.  Add the tests: section and limit the samples to build on
nrf52840dk_nrf52840 as that was the intended platform these samples
where initially developed for.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>