After QTest.keySequence(window, "Shift+A"), QTableWidget.selectRow (and selectColumn) are ineffective. The Python example code at PYSIDE-2407 shows the problem. A minimum example is roughly:

application = QApplication()
window = QMainWindow()
QTest.keySequence(window, "Shift+A")
table = QTableWidget(1, 1)
item = QTableWidgetItem()
table.setItem(0, 0, item)
table.selectRow(0)
assert table.selectedItems() == [item]

(I do not have a C++/Qt dev environment available.)