gdb, testsuite: fix gdb.base/save-history.exp for native-extended-gdbserver

When executed with the native-extended-gdbserver board file, we get

  FAIL: gdb.base/save-history.exp: check history contents

This is because the history file contains an extra line for the
command 'target extended-remote ...'.  Address this in the test.

Approved-By: Tom Tromey <tom@tromey.com>