test_plugin: examine mysql.user (#652)
test_plugin: examine mysql.user (#652)

In MySQL-8.0 the plugin is caching_sha2_password.

Test is fixed here by examining the plugin column from mysql.user
and comparing that to what was discovered during authentication.

MariaDB and MySQL-5.5 have a blank plugin column but still
report mysql_native_password in authentication.
PyMySQL/PyMySQL,pymysql/pymysql,methane/PyMySQL