Fix checking of warning messages

We produce 2 errors in this example.  We don't want to check that the
first one is present twice: we want to check that each warning is
present once.

Author: smortex

spec/puppet-lint/plugins/check_unsafe_interpolations_spec.rb

@@ -45,7 +45,7 @@ class foo {
 
       it 'creates two warnings' do
         expect(problems).to contain_warning(msg)
-        expect(problems).to contain_warning(msg)
+        expect(problems).to contain_warning("unsafe interpolation of variable 'bar' in exec command")
       end
     end