feat: parallel functional tests (~3x faster CI)

- Split 69 functional tests into 4 groups for parallel execution
- Add check-functional-parallel target to Makefile.am
- Fix VPATH builds with $(srcdir)/tests path
- Add test failure detection verification in CI
- Update Dockerfiles with parallel make -j$(nproc)
- Add --depth 1 for faster git clone in Dockerfiles

Dockerfile.tumbleweed

@@ -1,12 +1,16 @@
 # 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 \
   glib2-devel \