Tests don’t build

Discovered during [Debian packaging](https://github.com/AntiMicroX/antimicrox/issues/87), sorry for terseness, I’m short of time for this.

```
[ 97%] Building CXX object tests/CMakeFiles/GuiTests.dir/testaboutdialog.cpp.o
cd /tmp/buildd/antimicro-3.1.4/obj-x86_64-linux-gnu/tests && /usr/bin/c++ -DWITH_UINPUT -DWITH_X11 -DWITH_XTEST -I/tmp/buildd/antimicro-3.1.4/obj-x86_64-linux-gnu/tests -I/tmp/buildd/antimicro-3.1.4/tests -I/tmp/buildd/antimicro-3.1.4/obj-x86_64-linux-gnu/tests/GuiTests_autogen/include -I/tmp/buildd/antimicro-3.1.4/obj-x86_64-linux-gnu -I/tmp/buildd/antimicro-3.1.4/obj-x86_64-linux-gnu/src -I/tmp/buildd/antimicro-3.1.4 -I/tmp/buildd/antimicro-3.1.4/src -g -O2 -ffile-prefix-map=/tmp/buildd/antimicro-3.1.4=. -fstack-protector-strong -Wformat -Werror=format-security -Wno-deprecated-declarations -Wno-deprecated-copy -Wdate-time -D_FORTIFY_SOURCE=2 -I'/tmp/buildd/antimicro-3.1.4/debian/q-include' -std=c++11 -fPIE -o CMakeFiles/GuiTests.dir/testaboutdialog.cpp.o -c /tmp/buildd/antimicro-3.1.4/tests/testaboutdialog.cpp
In file included from /tmp/buildd/antimicro-3.1.4/tests/testaboutdialog.cpp:20:
/tmp/buildd/antimicro-3.1.4/src/aboutdialog.h:22:10: fatal error: QDialog: No such file or directory
22 | #include
| ^~~~~~~~~
compilation terminated.
make[2]: *** [tests/CMakeFiles/GuiTests.dir/build.make:98: tests/CMakeFiles/GuiTests.dir/testaboutdialog.cpp.o] Error 1
```

This is because it’s missing the Qt include directories; other files compile like this:

```
[ 96%] Building CXX object CMakeFiles/antimicrox.dir/src/eventhandlers/uinputeventhandler.cpp.o
/usr/bin/c++ -DQT_CONCURRENT_LIB -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DWITH_UINPUT -DWITH_X11 -DWITH_XTEST -I/tmp/buildd/antimicro-3.1.4/obj-x86_64-linux-gnu -I/tmp/buildd/antimicro-3.1.4 -I/tmp/buildd/antimicro-3.1.4/obj-x86_64-linux-gnu/src -I/tmp/buildd/antimicro-3.1.4/src -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -isystem /usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtWidgets -isystem /usr/include/x86_64-linux-gnu/qt5/QtConcurrent -isystem /usr/include/x86_64-linux-gnu/qt5/QtNetwork -g -O2 -ffile-prefix-map=/tmp/buildd/antimicro-3.1.4=. -fstack-protector-strong -Wformat -Werror=format-security -Wno-deprecated-declarations -Wno-deprecated-copy -Wdate-time -D_FORTIFY_SOURCE=2 -I'/tmp/buildd/antimicro-3.1.4/debian/q-include' -std=c++11 -fPIE -fPIC -o CMakeFiles/antimicrox.dir/src/eventhandlers/uinputeventhandler.cpp.o -c /tmp/buildd/antimicro-3.1.4/src/eventhandlers/uinputeventhandler.cpp
```

Compiling with `-DWITH_TESTS=OFF` works, so I’m using that for now, but I’d prefer for tests to run if there are tests, of course ☺