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.arch

@@ -36,8 +36,7 @@ RUN pacman -S --needed --noconfirm \
   sqlite \
   valgrind \
   gdk-pixbuf2 \
-  qrencode \
-  expect
+  qrencode
 
 RUN mkdir -p /usr/src/{stabber,profanity}
 
@@ -59,10 +58,6 @@ USER root
 
 RUN pacman -U --noconfirm libstrophe-git/libstrophe-git-*.pkg.tar.zst
 
-# Create symlink for libexpect - Arch puts it in versioned subdirectory
-# which causes configure to fail with "libexpect not found"
-RUN ln -sf /usr/lib/expect5.45.4/libexpect5.45.4.so /usr/lib/libexpect.so
-
 WORKDIR /usr/src
 RUN git clone -c http.sslverify=false https://git.jabber.space/devs/stabber