Rollup merge of #92238 - dtolnay:stringifytest, r=Mark-Simulacrum

Add a test suite for stringify macro

This attempts to cover the behavior of `stringify!` on various interpolated syntax tree nodes.

The pretty printer has a history of unsightly whitespace (double spaces, missing spaces, spaces where there shouldn't be spaces) — #91437, #91562, #91568. There are several such issues left; the test cases that I consider to be currently behaving incorrectly are marked with `// FIXME` in the PR.