MySQL method to find out view + fix fatal in tests

We were missing a method to list out views defined in a database. This
patch adds in MysqlBase::isView() and MysqlBse::listViews().

Since listViews() cache its result in DatabaseBase::$allViews, we also
introduce a final DatabaseBase::clearViewsCache() to let us clear the
per process cache.

Finally, fixed fatal error when duplicating VIEWs in MySQL.

bug: 43571
Change-Id: I8650baa4b721fe69ea3e1d557dd76745c0c7754e