Move /src/test to /tests

tests/mir-opt/const_prop/mutable_variable_unprop_assign.rs

@@ -0,0 +1,16 @@
+// unit-test
+// compile-flags: -O
+
+// EMIT_MIR mutable_variable_unprop_assign.main.ConstProp.diff
+fn main() {
+    let a = foo();
+    let mut x: (i32, i32) = (1, 2);
+    x.1 = a;
+    let y = x.1;
+    let z = x.0; // this could theoretically be allowed, but we can't handle it right now
+}
+
+#[inline(never)]
+fn foo() -> i32 {
+    unimplemented!()
+}