Redefining the constant like this produces a warning:
```
$ bundle exec rake
/Users/brandur/stripe/stripe-ruby/test/stripe/ephemeral_key_test.rb:75: warning: already initialized constant Stripe::EphemeralKeyTest::FIXTURE
/Users/brandur/stripe/stripe-ruby/test/stripe/ephemeral_key_test.rb:6: warning: previous definition of FIXTURE was here
Loaded suite /Users/brandur/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rake-11.1.2/lib/rake/rake_test_loader
Started
...
```
They also don't appear to be used, so it should be fine just to strip
them out of the test suite.
