crashes.

MySQL distributions contain a number of programs that are used only by
the MySQL test suite internally, i.e. they are not indended to be
invoked directly by a user. As a result, such programs are not
documented, do not have any built-in help or proper error reporting,
which may confuse users.

This patch fixes the problem with the following changes:
- mytest, libmysqltest and all references to them were removed from the
distribution since they are not used anymore
- bug25714 now displays an error message when run with incorrect
arguments or with the --help option
- mysql_client_test now does not call abort() in case of errors,
instead it does a clean exit() with a proper error status.