test: Change the headers! macro to a syntax that rustfmt will format

Rustfmt will format `()` or `[]` macro invocations that contain valid
Rust syntax, but not `{}` invocations. Switch to `()`, and update the
syntax to be valid Rust so rustfmt cleans them up.

(backport <#4765>)
(cherry picked from commit a8b1532)

Author: tgross35