Add unsafe extern blocks failing tests

Author: spastorino

tests/ui/rust-2024/unsafe-extern-blocks/extern-items.rs

@@ -0,0 +1,12 @@
+extern "C" {
+    static TEST1: i32;
+    fn test1(i: i32);
+}
+
+unsafe extern "C" {
+    //~^ ERROR: extern block cannot be declared unsafe
+    static TEST2: i32;
+    fn test2(i: i32);
+}
+
+fn main() {}

tests/ui/rust-2024/unsafe-extern-blocks/extern-items.stderr

@@ -0,0 +1,8 @@
+error: extern block cannot be declared unsafe
+  --> $DIR/extern-items.rs:6:1
+   |
+LL | unsafe extern "C" {
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+