vulkan: don't run tests or build lib if plugin isn't actually built
requested to merge tpm/gst-plugins-bad:disable-vulkan-tests-and-lib-if-vulkan-plugin-is-not-built into master
The unit tests only checked for vulkan_dep.found(), which can be true if the libs are there but glslc was not found, in which case the plugin wouldn't be built and the unit tests would fail because of missing vulkan plugins.
Doesn't really make much sense to build the vulkan integration lib either if we're not going to build the vulkan plugin, so just disable both for now if glslc is not available.
Fixes #1301 (closed)