Fix crash when using --check switch

[debonte]

Fix #73

Fixed regression that happened with the introduction of IssuesReport. The code that handles --check was still trying to call the old get_result_count_by_severity API.

• Fixed by using IssuesReport.get_issue_count_for_severity() instead.
• Removed dead SarifFile.get_result_count_by_severity code
• Added unit test.

[debonte]

@balgillo, if you have a minute, please take a look at this change. In particular I want to make sure that removing SarifFile.get_result_count_by_severity is the right move. It's clearly unused at the moment, but perhaps it shouldn't be?

Btw, I tried adding you explicitly with "Read" access in the repo settings to see if that would allow me to assign PRs to you. Despite your approval not being official, I value your feedback. It's not working at the moment, but perhaps you need to accept the invite that got sent to you.