AST Pretty-printer improvements and testing

[jpschorr]

Improve AST pretty-printer:

• Add DISTINCT for SELECT Clause
• Fix formatting of CASE (fixed by Improvements to pretty-printing for AND/OR/CASE #482)

Add AST pretty-print testing:

• snapshot testing against known outputs (fixed by Add snapshot testing for AST pretty printing #474 )
• round-trip testing ( -parse-> -pretty-print-> -parse-> ...) (fixed by Improvements to pretty-printing for AND/OR/CASE #482)
• fuzz AST generation?

Originally posted by @jpschorr in #471 (comment)