[gdb/testsuite] Handle missing curses in gdb.python/py-missing-debug.exp

On a system with python 3.6, module gdb.missing_debug imports module curses,
so when running test-case gdb.python/py-missing-debug.exp on a system without
that module installed, we run into:
...
(gdb) source py-missing-debug.py^M
Python Exception <class 'ImportError'>: Module 'curses' is not installed.^M
Use:^M
  sudo zypper install python36-curses^M
to install it.^M
Error occurred in Python: Module 'curses' is not installed.^M
Use:^M
  sudo zypper install python36-curses^M
to install it.^M
(gdb) FAIL: gdb.python/py-missing-debug.exp: source python script
...

Fix this by issuing UNSUPPORTED instead, and bailing out.

Tested on x86_64-linux.

Approved-by: Kevin Buettner <kevinb@redhat.com>

PR testsuite/31576
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31576