Add additional tests to serverSpec.js

This commit adds additonal tests to serverSpec.js test file. The
additional test are still testing the 'GET' functionality, but adds
tests for the '/api/atm' route and the '/api/atm/:atm_id' route.  All
tests are passing so far utilizing the provided data.json file.  I
expect to remove the dependency from the data file and have all data
mocked (correct term?) in the test itself.