Description:
X Plugin is located in optional directory, still its tests are located
in mysql-test/suite/xplugin. This suite should also be optional.

Solution:
Test suite was moved to rapid/plugin/x/tests/mtr/*. Right now X Plugin
needs to call INSTALL command in CMakeFile.txt (simillar like internal
plugins do) for its suite.

RB: 12602
Reviewed-by: Bjorn Munch <bjorn.munch@oracle.com>
Reviewed-by: Sayantan Dutta <sayantan.dutta@oracle.com>
Reviewed-by: Andrzej Religa <andrzej.religa@oracle.com>