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: #6490 (closed)