[autotests] Fix failing autotests

Summary:
After recent output-related changes testIdleInhibition and
testColorCorrectionNightColor started failing with the following output:

    ❯ dbus-run-session bin/testIdleInhibition
    ********* Start testing of TestIdleInhibition *********
    Config: Using QtTest library 5.13.0, Qt 5.13.0 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 9.1.0)
    QFATAL : TestIdleInhibition::initTestCase() ASSERT: "mode.size.isValid()" in file /home/vlad/Workspace/KDE/src/frameworks/kwayland/src/server/outputdevice_interface.cpp, line 188
    FAIL!  : TestIdleInhibition::initTestCase() Received a fatal error.
       Loc: [Unknown file(0)]
    Totals: 0 passed, 1 failed, 0 skipped, 0 blacklisted, 13ms
    ********* Finished testing of TestIdleInhibition ********

Apparently, they both don't initialize virtual outputs properly.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23539