TCK: NodeMixinUtil should exclude for mix:shareable

The mixin test NodeRemoveMixinTest#testRemoveSuccessfully tries to remove a mixin:

what it does: retrieve an addable mixin (NodeTypeUtil#getAddableMixinName), adds it and tries to remove it later on.
the addable mixins are retrieve from the complete set of mixin, testing node.canAddMixin.

However: with jackrabbit-core "mix:shareable" can be added but not removed.
if the test by chance gets exactly that mixin the test fails with exception (there is an explicit check the core for exactly that case).

the tck should exclude that special case, shouldn't it?

((michael found the issue))