maint: Fix order dependent test failure

When running spec/unit/network/rights_spec.rb prior to
spec/unit/network/authstore_spec.rb, test failures occur due to the
AuthStore storing Declarations in current thread state. This is a
temporary fix (explicitly clearing thread state for each test run) until
the AuthStore can be modified to not rely on thread state. This commit
is a follow up to 9834dcdd.

Paired-with: Jacob Helwig <jacob@puppetlabs.com>