Add various test cases, mostly xfailed

Author: catamorphism

src/test/compile-fail/issue-2951.rs

@@ -0,0 +1,9 @@
+// xfail-test
+fn foo<T, U>(x: T, y: U) {
+    let mut xx = x;
+    xx = y; // error message should mention T and U, not 'a and 'b
+}
+
+fn main() {
+    
+}

src/test/compile-fail/issue-2969.rs

@@ -0,0 +1,9 @@
+// xfail-test
+fn main()
+{
+// See #2969 -- error message should be improved
+   let mut x = [mut 1, 2, 4];
+   let v : &int = &x[2];
+   x[2] = 6;
+   assert *v == 6;
+}

src/test/compile-fail/issue-3080.rs

@@ -0,0 +1,9 @@
+// xfail-test
+enum x = ();
+impl x {
+    unsafe fn with() { } // This should fail
+}
+
+fn main() {
+    x(()).with();
+}

src/test/run-pass/issue-3154.rs

@@ -0,0 +1,11 @@
+struct thing<Q> {
+    x: &Q
+}
+
+fn thing<Q>(x: &Q) -> thing<Q> {
+    thing{ x: x }
+}
+
+fn main() {
+    thing(&());
+}