Call assume_migrated_upto_version on connection

Call assume_migrated_upto_version on connection to prevent it from first
being picked up in method_missing. In the base class, Migration,
method_missing expects the argument to be a table name, and calls
proper_table_name on the arguments before sending to connection. If
table_name_prefix or table_name_suffix is used, the schema version changes
to prefix_version_suffix, breaking `rake test:prepare`.

Fixes #10411.