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.arch

@@ -1,6 +1,7 @@
 FROM archlinux
 
 ENV TERM=xterm
+ENV CC="ccache gcc"
 
 RUN pacman -Syu --noconfirm
 # reflector is optional - if it fails due to network issues, continue with default mirrorlist
@@ -12,6 +13,7 @@ RUN pacman -S --needed --noconfirm \
   autoconf-archive \
   automake \
   base-devel \
+  ccache \
   check \
   cmake \
   cmocka \