Add test for E0796 error

tests/ui/static/use_of_mutable_static_unsafe_block.rs

@@ -0,0 +1,9 @@
+// compile-flags: --edition 2024 -Zunstable-options
+
+fn main() {
+    static mut X: i32 = 1;
+    unsafe {
+        let _y = &X;
+        //~^ ERROR use of mutable static is discouraged
+    }
+}

tests/ui/static/use_of_mutable_static_unsafe_block.stderr

@@ -0,0 +1,11 @@
+error[E0796]: use of mutable static is discouraged
+  --> $DIR/use_of_mutable_static_unsafe_block.rs:6:18
+   |
+LL |         let _y = &X;
+   |                  ^^ use of mutable static
+   |
+   = note: mutable statics can be mutated by multiple threads: aliasing violations or data races will cause undefined behavior
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0796`.

tests/ui/static/use_of_mutable_static_unsafe_fn.rs

@@ -0,0 +1,9 @@
+// compile-flags: --edition 2024 -Zunstable-options
+
+fn main() {}
+
+unsafe fn _foo() {
+    static mut X: i32 = 1;
+    let _y = &X;
+    //~^ ERROR use of mutable static is discouraged
+}

tests/ui/static/use_of_mutable_static_unsafe_fn.stderr

@@ -0,0 +1,11 @@
+error[E0796]: use of mutable static is discouraged
+  --> $DIR/use_of_mutable_static_unsafe_fn.rs:7:14
+   |
+LL |     let _y = &X;
+   |              ^^ use of mutable static
+   |
+   = note: mutable statics can be mutated by multiple threads: aliasing violations or data races will cause undefined behavior
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0796`.