Fix Signature2Test.

OpenJdk's behaviour around cloning Signature instances is well defined.
If the underlying SPI is not cloneable, Signature.clone() will always
throw. If the underlying SPI is cloneable, Signature.clone() will always
call through to the SPI's clone method. The return value of this
function does not depend on the state of the Signature object.

bug: 25925205
Change-Id: I6458bbf2fa5c76bf5e422227fbe51c9e21431348