Replace trace-related fixtures with factories.

The mocking and stubbing in the controller tests is
unfortunate, but the models interact directly with the filesystem
using the trace id so that's hard to control any other way.

test/factories/traces.rb

@@ -0,0 +1,16 @@
+FactoryGirl.define do
+  factory :trace do
+    sequence(:name) { |n| "Trace #{n}.gpx" }
+    sequence(:description) { |n| "This is trace #{n}" }
+
+    # Fixme requires User Factory
+    user_id 1
+
+    timestamp Time.now
+    inserted true
+
+    trait :deleted do
+      visible false
+    end
+  end
+end