Add tests for 'int_plus_one'

tests/ui/int_plus_one.rs

@@ -0,0 +1,18 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[allow(no_effect, unnecessary_operation)]
+#[warn(int_plus_one)]
+fn main() {
+    let x = 1i32;
+    let y = 0i32;
+    
+    x >= y + 1;
+    y + 1 <= x;
+
+    x - 1 >= y;
+    y <= x - 1;
+
+    x > y; // should be ok
+    y < x; // should be ok
+}

tests/ui/int_plus_one.stderr

@@ -0,0 +1,35 @@
+error: Unnecessary `>= y + 1` or `x - 1 >=`
+  --> $DIR/int_plus_one.rs:10:5
+   |
+10 |     x >= y + 1;
+   |     ^^^^^^^^^^
+   |
+   = note: `-D int-plus-one` implied by `-D warnings`
+   = help: Consider reducing `x >= y + 1` or `x - 1 >= y` to `x > y`
+
+error: Unnecessary `>= y + 1` or `x - 1 >=`
+  --> $DIR/int_plus_one.rs:11:5
+   |
+11 |     y + 1 <= x;
+   |     ^^^^^^^^^^
+   |
+   = help: Consider reducing `x >= y + 1` or `x - 1 >= y` to `x > y`
+
+error: Unnecessary `>= y + 1` or `x - 1 >=`
+  --> $DIR/int_plus_one.rs:13:5
+   |
+13 |     x - 1 >= y;
+   |     ^^^^^^^^^^
+   |
+   = help: Consider reducing `x >= y + 1` or `x - 1 >= y` to `x > y`
+
+error: Unnecessary `>= y + 1` or `x - 1 >=`
+  --> $DIR/int_plus_one.rs:14:5
+   |
+14 |     y <= x - 1;
+   |     ^^^^^^^^^^
+   |
+   = help: Consider reducing `x >= y + 1` or `x - 1 >= y` to `x > y`
+
+error: aborting due to 4 previous errors
+