Rename var -> variance for truncated distributions and close support.

[tillahoffmann]

• Rename var to variance for truncated distributions to match the base Distribution naming.
• Close the support for left/right truncated distribution to match the two-sided closed interval and other distributions with finite support, e.g., uniform, beta, etc.
• Include truncated distributions in mean/variance tests.
• Make mean/variance tests robust to NotImplementedErrors similar to other tests.

[juanitorduz]

thanks @tillahoffmann :)