[SyntaxTree] Split tests for expressions

We do that because:
* Big tests generated big tree dumps that could hardly serve as documentation.
* In most cases the tests didn't share setup, thus there was not much addition in lines of code.

We split tests for:
* `UserDefinedLiteral`
* `NestedBinaryOperator`
* `UserDefinedBinaryOperator`
* `UserDefinedPrefixOperator`
* `QualifiedId`

Differential Revision: https://reviews.llvm.org/D85819