dts: don't use 'test' vendor prefix

This is now failing an edtlib check for unknown vendor prefixes.

I can't find a reason to use a vendor prefix in application-local
bindings like this, so just remove it wherever it appears by
normalizing to test-foo-compat instead of test,foo_compat or
test,foo-compat.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>