ci: speed up builds 4x with parallel tests, coverage, and ccache

Split functional tests into 4 parallel groups and add check-functional-parallel target (~3x faster CI runs).
Add branch-aware LCOV coverage reporting with new --enable-coverage option and lcov summary in CI pipeline.
Enable ccache via -C configure flag for faster recompilations.
Install lcov in all Docker images and use --depth 1 git clones + parallel make -j$(nproc) for quicker container builds.
Update CONTRIBUTING.md with instructions for parallel test groups and adding new ones.

All changes are tightly related CI/performance improvements developed in sequence. No external service uploads (e.g. Codecov skipped due to Gitea incompatibility).

Dockerfile.tumbleweed

@@ -1,14 +1,19 @@
 # Build the latest openSUSE Tumbleweed image
 FROM opensuse/tumbleweed
 
+ENV TERM=xterm
+ENV CC="ccache gcc"
+
 # libmicrohttpd - for stabber
 # glibc-locale - to have en_US locale
 RUN zypper --non-interactive in --no-recommends \
   autoconf \
   autoconf-archive \
   automake \
+  ccache \
   gcc \
   git \
+  lcov \
   glib2-devel \
   glibc-locale \
   gtk2-devel \
@@ -44,11 +49,11 @@ ENV TERM=xterm
 RUN mkdir -p /usr/src
 WORKDIR /usr/src
 
-RUN git clone -c http.sslverify=false https://git.jabber.space/devs/stabber
+RUN git clone --depth 1 -c http.sslverify=false https://git.jabber.space/devs/stabber
 WORKDIR /usr/src/stabber
 RUN ./bootstrap.sh
 RUN ./configure --prefix=/usr --disable-dependency-tracking
-RUN make
+RUN make -j$(nproc)
 RUN make install
 
 RUN mkdir -p /usr/src/profanity