Add a few Snapshot tests

Author: Tom Johnson

lib/rdf/spec/mutable.rb

@@ -84,6 +84,16 @@
       end
     end
 
+    describe '#snapshot' do
+      it 'is implemented when #supports(:snapshots) is true' do
+        if mutable.supports?(:snapshots) 
+          expect(mutable.snapshot).to be_a RDF::Dataset
+        else
+          expect { mutable.snapshot }.to raise_error NotImplementedError
+        end
+      end
+    end
+
     context "when updating statements" do
       let(:s1) { RDF::Statement(resource, RDF::URI.new("urn:predicate:1"), RDF::URI.new("urn:object:1")) }
       let(:s2) { RDF::Statement(resource, RDF::URI.new("urn:predicate:2"), RDF::URI.new("urn:object:2")) }

lib/rdf/spec/repository.rb

@@ -77,6 +77,13 @@
       end
     end
 
+    it 'has repeatable read isolation or better' do
+      if repository.supports? :snapshots
+        good_isolation = [:repeatable_read, :snapshot, :serializable]
+        expect(good_isolation).to include repository.isolation_level
+      end
+    end
+
     it 'gives an accurate snapshot' do
       if repository.supports? :snapshots
         snap = repository.snapshot