test coverage increase for core

[Tlacenka]

In this issue, all coverage related subtasks from the test refactor are present.

• Add a happy path test to json-to-gql. Consider moving this file to utils.
• Add a test case for the no commit data available case in persist if not too difficult.
• The e2e test (snapshot) caught a additional property runner that slipped through when refactoring executePlugin.
  This should have been caught by one of the executePlugin tests. Add executePlugin tests #232
• Add test for PersistDirError in persist add test for throw PersistDirError in persist.spec.ts #220
• Add test for PersistError in persist add test for throw PersistError in persist.spec.ts #221
• Add test and better error for failed plugins.
  In this example the 3rd plugin failed. It would help to add the plugin slug at the top of the error.