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.fedora

@@ -1,6 +1,9 @@
 # Build the latest Fedora image
 FROM fedora:latest
 
+ENV TERM=xterm
+ENV CC="ccache gcc"
+
 # libmicrohttpd - for stabber
 # glibc-locale - to have en_US locale
 RUN dnf install -y \
@@ -8,6 +11,7 @@ RUN dnf install -y \
   autoconf-archive \
   automake \
   awk \
+  ccache \
   gcc \
   git \
   glib2-devel \