test generators should use post-re-organisation x-common JSON file paths

[petertseng]

exercism/problem-specifications#351 has reorganised the JSON filenames in the x-common repo. See https://github.com/exercism/xgo/blob/master/exercises/leap/example_gen.go for an example of Gen being used called with leap.go, and it appears Gen in https://github.com/exercism/xgo/blob/master/gen/gen.go will expect that filename. Before the reorganisation, this file was located at x-common/leap.json. After the reorganisation, this file is located in e.g. x-common/exercises/leap/canonical-data.json .

This means that if I were to run for example go run example_gen.go right now it would not be able to find the file.

The task here is:

• Make Gen search for the file in the correct place
• Change all callers of Gen, if necessary, depending on the implementation of the above. A suggestion I have is that Gen need only take the exercise slug such as "leap"

After this, we should be able to run go run example_gen.go again.