New tests for the Pangram problem

[IanWhitney]

We have found that the Pangram tests miss edge cases allowing students to pass all of the current tests with an incorrect implementation.

To cover these cases we have added new tests to the Pangram test set. Those new tests were added in this commit

Since this track implements Pangram, please take a look at the new pangram.json file and see if your track should update its tests.

If you do need to update your tests, please refer to this issue in your PR. That helps us see which tracks still need to update their tests.

If your track is already up to date, go ahead and close this issue.

More details on this change are available in x-common issue 222.

Thank you for your help!