NotificationsSpec: Replace hardcoded repo name with @test_repo
NotificationsSpec: Rename "methods that need a thread context" context

to "with thread"
NotificationsSpec: Add "with subscription" context with setup

Add context and setup block to create a subscription and moved the specs
requiring it inside the context.
Milestones: Replace hardcoded repo name with @test_repo
Milestones: Rename "methods that need a milestone" context to "with milestone"
Issues: Replace hardcoded repo name with @test_repo
Issues: Rename "methods requiring a new issue" context to "with issue"
Issues: Rename "methods that reqire a new issue comment" context to
"with issue comment"
Add OCTOKIT_TEST_GITHUB_ORGANIZATION ENV to set test org name.
Issues: Replace hardcoded organization name with test_github_org helper