ha_innodb.cc: Declare strict_mode as PLUGIN_VAR_OPCMDARG, because we
do want to be able to disable innodb_strict_mode.  This is a non-functional
change, because PLUGIN_VAR_NOCMDARG seems to accept an argument as well.

innodb-zip.test: Do not store innodb_strict_mode.  It is a session variable.
Add a test case for innodb_strict_mode=off.