Fix LinterRegistry specs to not pollute actual registry

In the process of writing some other specs, there was a problem where
the linters inserted into the registry by this spec were breaking other
specs.

Fix by modifying the specs to not modify the global linter registry.

Change-Id: I22c0e282003da4f6ebc8f52758654f338e64bd5f
Reviewed-on: https://gerrit.causes.com/30918
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>