[FEA] Migrate Unit tests to use AST Tree

[lamarrr]

Is your feature request related to a problem? Please describe.
We presently manually construct and manage the lifetimes of AST tree subexpressions, the ast subexpressions bind references to each other, we added an AST tree type (ast_tree) in #17156 to help with this and also automatically cleanup the objects.
Our tests manually construct the AST subexpression trees which is error-prone and has lead to bugs (i.e. #17292).

Describe the solution you'd like
We'd like to convert our unit tests and benchmarks to use the new ast_tree type, and also recommend it to our users instead of manually constructing the tree.