Allow single test file to run without failing

[hovsater]

Before, running single test files dependent on the User test class would fail if spec/formulaic/label_spec.rb was not run first.

This commit addresses the issue by requiring test classes in the spec/spec_helper.rb instead. Now all test files can be run independently.

[alex-ross]

@calebthompson any chance of merging this?

[calebhearth]

Yup!