assertions: use assert_eq! where possible

Using `assert_eq!` instead of `assert!` has the advantage that when the
assertion fails, the debug representation of both sides will be shown,
which can provide more information about the failure than only seeing
that the assertion failed.

Part of #12336

crates/printf/src/lib.rs

@@ -84,7 +84,7 @@ macro_rules! sprintf {
 ///
 /// let result = printf_c_locale(&mut output, fmt, &mut args);
 ///
-/// assert!(result == Ok(10));
+/// assert_eq!(result, Ok(10));
 /// assert_eq!(output, "1.2346e+05");
 /// ```
 pub fn printf_c_locale(