Add a test for explicit i suffix on integer literals

This doesn't seem to show up anywhere else in the test suite, even
though it's supposed to be legal.

src/test/run-pass/explicit-i-suffix.rs

@@ -0,0 +1,9 @@
+fn main() {
+    let x: int = 8i;
+    let y = 9i;
+    x + y;
+
+    let q: int = -8i;
+    let r = -9i;
+    q + r;
+}