build(docker): enable stabber for functional tests

- Enable stabber build from https://git.jabber.space/devs/stabber
- Add tcl-expect-dev package for Debian/Ubuntu (required by libexpect)
- Add expect package for Arch Linux
- Fix libexpect detection in configure.ac (requires -ltcl8.6 on Debian/Ubuntu)
- Remove stabber submodule, use system-installed libstabber instead
- Update Makefile.am to use system libstabber paths

configure.ac

@@ -375,8 +375,12 @@ AM_CONDITIONAL([HAVE_STABBER], [false])
 AC_CHECK_LIB([stabber], [stbbr_start], [AM_CONDITIONAL([HAVE_STABBER], [true])],
     [AC_MSG_NOTICE([libstabber not found, will not be able to run functional tests])])
 AM_CONDITIONAL([HAVE_EXPECT], [false])
+dnl libexpect on Debian/Ubuntu requires -ltcl8.6 for linking
+saved_LIBS="$LIBS"
+LIBS="$LIBS -ltcl8.6"
 AC_CHECK_LIB([expect], [exp_expectl], [AM_CONDITIONAL([HAVE_EXPECT], [true])],
     [AC_MSG_NOTICE([libexpect not found, will not be able to run functional tests])])
+LIBS="$saved_LIBS"
 
 ## Default parameters
 AM_CFLAGS="$AM_CFLAGS -Wall -Wno-deprecated-declarations -std=gnu99 -ggdb3"