misc: use BUILD_TESTING like the daemon and update instructions

CMakeLists.txt

INSTALL.md

build.py

extras/ci/client-qt-gnulinux/Jenkinsfile

extras/scripts/build-windows.py

extras/scripts/install.sh

src/app/main.cpp

src/app/utilsadapter.cpp

src/app/utilsadapter.h

src/libclient/CMakeLists.txt

tests/CMakeLists.txt