clock.json: test data for clock

This is a proposal to store nominal test cases for exercises in x-common.  Initial discussion is at https://github.com/exercism/xgo/issues/114.

Currently when someone codes the test program for an exercise in a new language, they typically port test cases from another language, often Ruby.  Test programs then diverge over time.  When an Improvement is made in one language, nothing drives the improvement to other languages.  With a common repository for test cases, we would have a place to share improvements.

With data in a standard format like JSON however, there is the possibility of programmatically extracting test cases for use in native language test programs.  https://github.com/exercism/xgo/tree/clock-json/clock for example, has code reading this JSON file and converting it to Go native data which is then included with the test program.  JSON is well supported and should be accessible from most programming languages.
The previous comment has an array of strings, whereas this one has just a string. I think we should be consistent.