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

tests/functionaltests/test_software.c

@@ -4,7 +4,6 @@
 #include <string.h>
 
 #include <stabber.h>
-#include <expect.h>
 
 #include "proftest.h"