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.debian

@@ -8,7 +8,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   autoconf \
   autoconf-archive \
   automake \
-  expect \
   gcc \
   git \
   libcmocka-dev \
@@ -33,8 +32,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   valgrind \
   libsqlite3-dev \
   libgdk-pixbuf-2.0-dev \
-  libqrencode-dev \
-  tcl-expect-dev
+  libqrencode-dev
 
 RUN mkdir -p /usr/src/{stabber,libstrophe,profanity}
 WORKDIR /usr/src