Skip synthetic updater tests gracefully when optional KDE modules like
org.kde.plasma.plasma5support are not available locally, aligning with
the existing tst_*.qml test suites. Maintain strict assertions when
QML_TEST_REQUIRE_NO_SKIPS=1 is set in CI.

Validation:
- make check passed locally without errors.
- QML_TEST_REQUIRE_NO_SKIPS=1 fails as expected when modules are missing.
- unittest test_widget_update_controller passed with skipped=1 locally.