In JSON, the colon actually is used for assignment, but in spa_type_info
the colon is used as a separator. To properly parse and pass the
properties to the videotestsrc, the key has to be enclosed by quotation
marks to prevent splitting by the JSON parser.
Commit 9b29bcd4e8 ("conf: set description for test nodes") set the
description only for the minimal.conf and pipewire.conf and skipped the
pipewire-vulkan.conf.
Set the description for pipewire-vulkan.conf, too.
