Fix gdb.server/server-exec-info.exp with the extended-remote board

Fix gdb.server/server-exec-info.exp with the extended-remote board

This test fails with --target_board=native-extended-gdbserver because
it misses the usual "disconnect":

 (gdb) spawn ../gdbserver/gdbserver --once :2347 /home/pedro/gdb/mygit/build/gdb/testsuite/gdb.server/server-exec-info
 Process /home/pedro/gdb/mygit/build/gdb/testsuite/gdb.server/server-exec-info created; pid = 4736
 Listening on port 2347
 target extended-remote localhost:2347
 Already connected to a remote target.  Disconnect? (y or n) ^CsQuit
 (gdb) et sysroot remote:
 Undefined command: "et".  Try "help".
 (gdb) n
 The program is not being run.
 (gdb) FAIL: gdb.server/server-exec-info.exp: set sysroot remote: (got interactive prompt)
 info files
 (gdb) FAIL: gdb.server/server-exec-info.exp: info files

gdb/testsuite/ChangeLog:
2015-07-28  Pedro Alves  <palves@redhat.com>

	* gdb.server/server-exec-info.exp: Issue a "disconnect".