Only connect to qApp in init and disconnect in cleanup.
Then don't call cleanup in NoUi tests.

Fixes UBSAN report:
```
/home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/shell/uicontroller.cpp:241:16: runtime error: downcast of address 0x7f30c868a1c0 which does not point to an object of type 'QApplication'
0x7f30c868a1c0: note: object is of type 'QCoreApplication'
 00 00 00 00  f0 4f 59 de 30 7f 00 00  e0 55 02 00 e0 60 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'QCoreApplication'
    #0 0x7f30eeacada3 in KDevelop::UiController::~UiController() /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/shell/uicontroller.cpp:241
    #1 0x7f30eeacb874 in KDevelop::UiController::~UiController() /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/shell/uicontroller.cpp:242
    #2 0x7f30eea745d3 in KDevelop::CorePrivate::~CorePrivate() /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/shell/core.cpp:279
    #3 0x7f30eea7f4be in KDevelop::Core::~Core() /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/shell/core.cpp:330
    #4 0x7f30f1250435 in KDevelop::TestCore::~TestCore() kdevplatform/tests/KDevPlatformTests_autogen/EWIEGA46WW/../../../../../../../../src/extragear/kdevelop/kdevelop/kdevplatform/tests/testcore.h:75
    #5 0x7f30f1250435 in KDevelop::TestCore::~TestCore() kdevplatform/tests/KDevPlatformTests_autogen/EWIEGA46WW/../../../../../../../../src/extragear/kdevelop/kdevelop/kdevplatform/tests/testcore.h:75
    #6 0x7f30de32a33f in QObject::event(QEvent*) (/usr/lib/libQt5Core.so.5+0x2e233f)
    #7 0x7f30de2fda4f in QCoreApplication::notifyInternal2(QObject*, QEvent*) (/usr/lib/libQt5Core.so.5+0x2b5a4f)
    #8 0x7f30de300572 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (/usr/lib/libQt5Core.so.5+0x2b8572)
    #9 0x7f30de3522dd in QTest::qWait(int) (/usr/lib/libQt5Core.so.5+0x30a2dd)
    #10 0x7f30f1279cef in KDevelop::TestCore::shutdown() /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/tests/testcore.cpp:99
```