Cache the fixture set filenames that have already been loaded

Test frameworks such as RSpec include ActiveRecord::TestFixtures for each individual test suite class, and since they generate one such test suite class for each 'describe' or 'context' call, there are many such classes in a full suite.

The fixtures method is therefore called many times in a large test file/set.  It calls require_fixture_classes, which calls singularize and try_to_load_dependency/require_dependency for each fixture configured, both of which are quite slow, making this repetition a major slow-down - adding several seconds to the start-up time for some of our test files.

This patch improves the load time of large files by 88% in my testing.  It simply memoizes the list of fixture files already put through the singularize/try_to_load_dependency loop (using a module-level accessor so that it persists across those multiple test classes).  Since requires are global, there's no need to repeat this work for each test class.