Change gdb test suite's TERM setting

Change gdb test suite's TERM setting

This changes the gdb test suite to set TERM to "dumb" by default.
This setting disables terminal styling, so that the existing tests do
not need to be updated.

gdb/testsuite/ChangeLog
2018-12-28  Tom Tromey  <tom@tromey.com>

	* lib/gdb.exp (gdb_init): Set the TERM environment variable to
	"dumb".
	* gdb.base/readline.exp (operate_and_get_next): Save and restore
	the TERM environment variable.