Implement QTest::qEnsureLeftCorner() to set and ensure window's position to the left corner

Implement QTest::qEnsureLeftCorner() to set and ensure the window's position to the left corner of the screen.

Many cases of flakiness observed on Opensuse are due to un-updated position and frameposition of a window. This flakiness can be fixed by assigning a fixed position and verifying that the window's position corresponds to it.

The helper test function first sets a position using the available top-left corner of the screen and then verifies that the position and framePosition have been updated.

List of flaky tests where that function could be used:

• Opensuse