Summary:
Adds an autotest to show that KWin fails an assertion when a client tries to
resize a sub-surface.

Since it is the first autotest dealing with sub-surfaces explicitly additional
autotest helpers are introduced to allow that.

We also add a new signal in Compositor to spy on to know when the buffer swap
has been completed.

Test Plan:
Test fails as expected:
```
QFATAL : KWin::BufferSizeChangeTest::testShmBufferSizeChangeOnSubSurface() ASSERT: "image.size() == m_size" in file /home/roman/dev/kde/src/kde/workspace/kwin/platformsupport/scenes/opengl/abstract_egl_backend.cpp, line 394
FAIL!  : KWin::BufferSizeChangeTest::testShmBufferSizeChangeOnSubSurface() Received a fatal error.
   Loc: [Unknown file(0)]
Totals: 4 passed, 1 failed, 0 skipped, 0 blacklisted, 367ms
********* Finished testing of KWin::BufferSizeChangeTest *********
```

Reviewers: #kwin, zzag

Subscribers: zzag, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18452