feat: add code coverage with branch coverage

- Add --enable-coverage configure option
- Add lcov targets for generating coverage reports
- Enable branch coverage (--rc branch_coverage=1)
- Install lcov in all Docker images
- Coverage report displayed via lcov --summary (no Codecov - doesn't work with Gitea)

Dockerfile.arch

@@ -17,6 +17,7 @@ RUN pacman -S --needed --noconfirm \
   check \
   cmake \
   cmocka \
+  lcov \
   curl \
   debuginfod \
   doxygen \