tests: activate misspelled gdb-check in function-arg-initialization.rs

In 9253e12 a bunch of `gdbr-check` (for `rust-gdb`) and
`gdbg-check` (for plain `gdb`) was added. But in two places the author
accidentally wrote `gdbt-check` instead. This commit fixes this typo.

Note that we need to add a `std::hint::black_box()` to avoid

    $4 = <optimized out>

prints on at least `aarch64-gnu-llvm-20-1`.

Author: Enselic

tests/debuginfo/function-arg-initialization.rs

@@ -26,9 +26,9 @@
 
 // NON IMMEDIATE ARGS
 // gdb-command:print a
-// gdbt-check:$4 = function_arg_initialization::BigStruct {a: 3, b: 4, c: 5, d: 6, e: 7, f: 8, g: 9, h: 10}
+// gdb-check:$4 = function_arg_initialization::BigStruct {a: 3, b: 4, c: 5, d: 6, e: 7, f: 8, g: 9, h: 10}
 // gdb-command:print b
-// gdbt-check:$5 = function_arg_initialization::BigStruct {a: 11, b: 12, c: 13, d: 14, e: 15, f: 16, g: 17, h: 18}
+// gdb-check:$5 = function_arg_initialization::BigStruct {a: 11, b: 12, c: 13, d: 14, e: 15, f: 16, g: 17, h: 18}
 // gdb-command:continue
 
 // BINDING
@@ -234,6 +234,7 @@ struct BigStruct {
 
 fn non_immediate_args(a: BigStruct, b: BigStruct) {
     zzz(); // #break
+    std::hint::black_box(|| { let (_, _) = (a, b); }); // Avoid `<optimized out>` prints
 }
 
 fn binding(a: i64, b: u64, c: f64) {