wallpapers/image: add basic unit test for frontend
This adds 3 tests for the frontend, each with different fill modes and background colors.
- Load image with the default settings
- Load image with blur enabled and PreserveAspectFit fill mode
- Load image with Pad fill mode
This uses QQuickItem::grabImage to check the color is coorect in each test case.
Local test result:
> ./testimagefrontend
********* Start testing of ImageFrontendTest *********
Config: Using QtTest library 5.15.7, Qt 5.15.7 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 12.2.1 20221020 [revision 0aaef83351473e8f4eb774f8f999bbe87a4866d7]), opensuse-tumbleweed 20221111
PASS : ImageFrontendTest::initTestCase()
QWARN : ImageFrontendTest::testLoadWallpaper(Default) Could not create scene graph context for backend 'automaticbackend' - check that plugins are installed correctly in /usr/lib64/qt5/plugins
PASS : ImageFrontendTest::testLoadWallpaper(Default)
PASS : ImageFrontendTest::testLoadWallpaper(Blur enabled)
PASS : ImageFrontendTest::testLoadWallpaper(Background color)
PASS : ImageFrontendTest::cleanupTestCase()
Totals: 5 passed, 0 failed, 0 skipped, 0 blacklisted, 739ms
********* Finished testing of ImageFrontendTest *********