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).

configure.ac

@@ -69,6 +69,8 @@ AC_ARG_ENABLE([gdk-pixbuf],
     [AS_HELP_STRING([--enable-gdk-pixbuf], [enable GDK Pixbuf support to scale avatars before uploading])])
 AC_ARG_ENABLE([omemo-qrcode],
     [AS_HELP_STRING([--enable-omemo-qrcode], [enable ability to display omemo qr code])])
+AC_ARG_ENABLE([coverage],
+    [AS_HELP_STRING([--enable-coverage], [enable code coverage analysis])])
 
 m4_include([m4/ax_valgrind_check.m4])
 AX_VALGRIND_DFLT([drd], [off])
@@ -386,6 +388,11 @@ AC_SUBST([FORKPTY_LIB])
 AM_CFLAGS="$AM_CFLAGS -Wall -Wno-deprecated-declarations -std=gnu99 -ggdb3"
 AM_LDFLAGS="$AM_LDFLAGS -export-dynamic"
 
+AS_IF([test "x$enable_coverage" = xyes],
+    [AM_CFLAGS="$AM_CFLAGS --coverage -O0"
+     AM_LDFLAGS="$AM_LDFLAGS --coverage"
+     AC_MSG_NOTICE([Code coverage analysis enabled])])
+
 AS_IF([test "x$PACKAGE_STATUS" = xdevelopment],
     [AM_CFLAGS="$AM_CFLAGS -Wunused -Werror"])
 AS_IF([test "x$PLATFORM" = xosx],