[test] webcompat/issues.py needs more tests coverage

[karlcow]

coverage is missing currently

Name                                   Stmts   Miss  Cover   Missing
--------------------------------------------------------------------
webcompat/issues.py                       48      4    92%   99-104

webcompat.com/webcompat/issues.py

Lines 99 to 104 in 54a27f0

	abort(400)
	elif (not proxy) and submit_type == 'github-auth-report':
	# returns JSON data as a dict
	json_response = github.post(path, build_formdata(form))
	else:
	abort(400)

Probably two steps.

• convert to pytest (see this diff as an example and documentation
• add the remaining missing tests scenario

the file to edit is
https://github.com/webcompat/webcompat.com/blob/master/tests/unit/test_issues.py