add remote track tests

- Added remote track data tests for firstdata_e4 gateway 
- Tests pass successfully 

@j-mutter @jnormore @louiskearns 
This ends up sending a `credit_card` with both track data *and* [fully populated manual-entry data](https://github.com/Shopify/active_merchant/blob/firstdata_track_remote_tests/test/test_helper.rb#L150-L162). I think this should be a `Billing::CreditCard` with *only* track data, no?