auth: Move GitHub auth tests out of SocialAuthBase.
auth: Move GitHub auth tests out of SocialAuthBase.

During the time between when we refactored the GitHub authentication
backend to use SocialAuthBase and now (when we're about to migrate
GoogleAuthBackend to use that code path as well), we accidentally
added some GitHub-specific authentication backend tests to the common
test class.

Fix this by moving them to the GitHub-specific subclass.

timabbott/zulip,showell/zulip,shubhamdhama/zulip,synicalsyntax/zulip,kou/zulip,zulip/zulip,synicalsyntax/zulip,rht/zulip,hackerkid/zulip,timabbott/zulip,tommyip/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,shubhamdhama/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,shubhamdhama/zulip,hackerkid/zulip,synicalsyntax/zulip,shubhamdhama/zulip,shubhamdhama/zulip,brainwane/zulip,tommyip/zulip,zulip/zulip,brainwane/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,timabbott/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,brainwane/zulip,tommyip/zulip,hackerkid/zulip,brainwane/zulip,synicalsyntax/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,brainwane/zulip,rht/zulip,showell/zulip,synicalsyntax/zulip,zulip/zulip,showell/zulip,showell/zulip,kou/zulip,punchagan/zulip,zulip/zulip,shubhamdhama/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,timabbott/zulip,rht/zulip,andersk/zulip,kou/zulip,zulip/zulip,punchagan/zulip,timabbott/zulip,synicalsyntax/zulip,andersk/zulip,eeshangarg/zulip,tommyip/zulip,andersk/zulip,rht/zulip,showell/zulip,kou/zulip,punchagan/zulip,showell/zulip,brainwane/zulip,timabbott/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,tommyip/zulip,shubhamdhama/zulip,tommyip/zulip,tommyip/zulip