Unit test for convert_json_to_tibble()

[btskinner]

Add unit tests for convert_json_to_tibble() in utility_functions.R file.

• Create test-json.R in ./tests/testthat
• Create fake json string at top of file (or pull from small call)
  • Single level
  • Nested (like new program data in Scorecard data)
• Tests to confirm proper conversion to tibble