tests: activate misspelled `gdb-check` in `function-arg-initialization.rs`
In 9253e1206e a bunch of `gdbr-check` (for `rust-gdb`) directives and `gdbg-check` (for plain `gdb`) directives were added. But in two places the author accidentally wrote `gdbt-check` instead (`t` is next to `r` on the keyboard). This commit fixes that typo.
rust-lang/rust#129218 later renamed `gdbr-check` to just `gdb-check` which is why we rename to `gdb-check` directly.
The test still passes locally for me after the change, but fails if I change the `gdb-check` checks to check for some other string, so the check seems to still perform its intended function.
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`.
After this there are no more instances of the string `gdbt` in the code base:
```console
$ git grep gdbt
```
try-job: dist-i586-gnu-i586-i686-musl
35ebdf9ba1