fix(tests): replace libexpect with forkpty() for functional tests

Remove dependency on libexpect which had a segfault bug on Arch Linux.

- Replace exp_spawnl() with forkpty() for PTY creation
- Replace exp_expectl() with custom POSIX regex matching
- Update configure.ac to check for forkpty() instead of libexpect
- Update Makefile.am to link with libutil instead of libexpect/libtcl
- Remove expect/tcl packages from all Dockerfiles
- Add Valgrind suppressions for stabber/pthread false positives

Dockerfile.fedora

@@ -1,7 +1,6 @@
 # Build the latest Fedora image
 FROM fedora:latest
 
-# expect - for functional tests
 # libmicrohttpd - for stabber
 # glibc-locale - to have en_US locale
 RUN dnf install -y \
@@ -9,7 +8,6 @@ RUN dnf install -y \
   autoconf-archive \
   automake \
   awk \
-  expect-devel \
   gcc \
   git \
   glib2-devel \