Update oembed_test to work with new OEmbed HTML response

The OEmbed test (`oembed_test.go`) was failing due to a change in the hashtag link returned by the Twitter API. This updates the expected HTML so that the test passes again.

It also moves the test into the `anaconda_test` package, as the test did not run properly when executed with `go test oembed_test.go` (rather than `go test`).