The signal is declared in GtkTestATContext with 0 parameters, but these
handlers were written as if the signal had one `guint` parameter.
On some architectures this accidentally works as intended, but on
others (reproduced on i386 and riscv64) the test tries to use arbitrary
stack contents as the `TestData *` and crashes when it tries to
dereference the resulting non-pointer.
Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/6490
Signed-off-by: Simon McVittie <smcv@debian.org>