fix typo during interactive_mode check in gdb_has_a_terminal

fix typo during interactive_mode check in gdb_has_a_terminal

Discovered by Pierre Muller.

gdb/ChangeLog:

        * inflow.c (gdb_has_a_terminal): Fix typo in interactive_mode
        value test.

gdb/testsuite/ChangeLog:

        * gdb.base/interact.exp: Add extra tests that verify that
        the value of the interactive-mode setting does not change
        after the script is sourced.