Currently when we add a new CDM version support, we update ClearKeyCdm to support that new version.

This causes a problem now because mojo CDM would support the new version, but we have no plan to update pepper CDM to support it. But if ClearKeyCdm only supports one version, we are stuck.

Long term, it is also possible that we work on the new experimental CDM interface, while we still ship the old (stable) CDM interface in production. Hence, we still want to keep the test coverage of the old CDM interface while we work on the new one.

Thus, it makes some sense if we can support running our tests on both the old and new CDM interface. This would require us to support two versions of CDM interfaces in the same ClearKeyCdm.