It depends on version of Metadata API. Here is a list of all metadata, that causes test execution before 34.0.
If the deployment includes components for the following metadata
types, all local tests are run.
- Validation Rules
If Metadata API is version 34.0 and higher, only deployment of Apex class or Apex trigger causes deployment test execution.
When deploying to production, all tests, except those that originate
from managed packages, are executed if your deployment package
contains Apex classes or triggers. If your package doesn’t contain
Apex components, no tests are run by default.