Use save_vars to replace existing manipulation of globals in tests

Use save_vars to replace existing manipulation of globals in tests

gdb/testsuite/ChangeLog:

	* gdb.base/gdbhistsize-history.exp
	(test_histsize_history_setting): Use save_vars.
	* gdb.base/gdbinit-history.exp (test_gdbinit_history_setting):
	Use save_vars.
	(test_no_truncation_of_unlimited_history_file): Use save_vars.
	* gdb.base/readline.exp: Use save_vars.