Tests for third_party code are run out of the main workspace.  This
isn't an issue when the main workspace is the
com_github_google_flatbuffers workspace, but is an issue when you are
running the tests from another repository.

To reproduce, use "git_repository" to add flatbuffers to a project and
then run:
  bazel test @com_github_google_flatbuffers//:flatbuffers_test