Add binary operator ensure tests

Currently failing:

    error: unexpected token: `b`
       --> tests/test_ensure.rs:138:44
        |
    138 |     let test = || Ok(ensure!(a <= b || a - b <= 10));
        |                                            ^
        |
       ::: src/ensure.rs:319:117
        |
    319 |     (atom $stack:tt $bail:tt (~$($fuel:tt)*) {($($buf:tt)*) $($parse:tt)*} ($dot:tt $ident:tt $colons:tt $larrow:tt $lit:literal $($dup:tt)*) . $i:ident :: <- $($rest:tt)*) => {
        |                                                                                                                     ------------ while parsing argument for this `literal` macro fragment

tests/test_ensure.rs

@@ -132,6 +132,19 @@ fn test_low_precedence_binary_operator() {
         test,
         "Condition failed: `while false == true && false {} < ()` (() vs ())",
     );
+
+    let a = 15;
+    let b = 3;
+    let test = || Ok(ensure!(a <= b || a - b <= 10));
+    assert_err(test, "Condition failed: `a <= b || a - b <= 10`");
+}
+
+#[test]
+fn test_high_precedence_binary_operator() {
+    let a = 15;
+    let b = 3;
+    let test = || Ok(ensure!(a - b <= 10));
+    assert_err(test, "Condition failed: `a - b <= 10` (12 vs 10)");
 }
 
 #[test]