Assert a difference is equal to an expression.
Pseudocode: Δ = x
use assertables::*; let a: i8 = 10; let b: i8 = 13; let x: i8 = 3; assert_diff_eq_x!(a, b, x);
assert_diff_eq_x
assert_diff_eq_x_as_result
debug_assert_diff_eq_x