Make backup test work with MySQL 5.7. (#2442)
Make backup test work with MySQL 5.7. (#2442)

MySQL 5.7 doesn't have "password" field in mysql.user table anymore, it uses
"authentication_string" to store the password hash. I'm fixing the test to
determine whether the "password" field exists in mysql.user and then to use
either "password" or "authentication_string" depending on what exists in the
used MySQL version.

BUG=34071871
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