tests: Fix signature of GtkTestATContext::update-selection-bound handlers

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>

testsuite/a11y/text.c

@@ -45,7 +45,6 @@ update_caret_pos (GtkATContext *context,
 
 static void
 update_selection_bound (GtkATContext *context,
-                        guint         selection_bound,
                         gpointer      data)
 {
   TestData *td = data;

testsuite/a11y/textview.c

@@ -72,7 +72,6 @@ update_caret_pos (GtkATContext *context,
 
 static void
 update_selection_bound (GtkATContext *context,
-                        guint         selection_bound,
                         gpointer      data)
 {
   TestData *td = data;