Remove iotty test

Remove iotty test

The auxiliary utility ttyemu proved to be unreliable.  Given existing
differences between pty implementations and termios ioctls on various
platforms, writing it in a portable way requires effort disproportional
to its actual purpose.

* configure.ac: Remove check for grantpt
* gnulib.modules: Remove posix_openpt, ptsname, and unlockpt
* tests/Makefile.am (TESTSUITE_AT): Remove iotty.at
(check_PROGRAMS): Remove ttyemu.
* tests/testsuite.at: Remove iotty.at
* tests/iotty.at: Remove.
* tests/ttyemu.c: Remove.