Add coverage report generation

[sikmir]

How to use (with devcontainer):

(cd build; cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_INSTALL_SYSCONFDIR=/etc -DCMAKE_INSTALL_LOCALSTATEDIR=/var -DCMAKE_EXPORT_NO_PACKAGE_REGISTRY=ON -DCMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY=ON "-GUnix Makefiles" -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_SYSTEM_NAME=Linux -DCMAKE_SYSTEM_PROCESSOR=armv7l -DCMAKE_C_COMPILER=arm-linux-gnueabihf-gcc -DCMAKE_CXX_COMPILER=arm-linux-gnueabihf-g\\+\\+ -DPKG_CONFIG_EXECUTABLE=/usr/bin/arm-linux-gnueabihf-pkg-config -DPKGCONFIG_EXECUTABLE=/usr/bin/arm-linux-gnueabihf-pkg-config -DQMAKE_EXECUTABLE=/usr/bin/arm-linux-gnueabihf-qmake -DCMAKE_INSTALL_LIBDIR=lib/arm-linux-gnueabihf -DCMAKE_BUILD_TYPE=Debug ..)
(cd build; make test)
gcovr --html build/cov.html build
open build/cov.html

Using wbdev:

DEB_BUILD_OPTIONS="debug check cov-fail-under=79 cov-report=cov.html" ./wbdev cdeb