Added missing test files

Author: FractalFir

test/empty_string_slice.rs

@@ -0,0 +1,4 @@
+pub fn main(){
+    let slice = "";
+    std::hint::black_box(slice);
+}

test/fn_ptr.rs

@@ -0,0 +1,17 @@
+#[allow(improper_ctypes_definitions)]
+#[no_mangle]
+pub extern fn test_fn(a:fn (i32)->i32)->fn (i32)->i32{
+    a
+}
+#[allow(improper_ctypes_definitions)]
+#[no_mangle]
+pub extern fn test_fn2(a:i32)->i32{
+    a
+}
+#[allow(improper_ctypes_definitions)]
+#[no_mangle]
+pub extern fn return_fn()->fn (i32)->i32{
+    fn simple(a:i32)->i32{a}
+    simple
+}
+