Add OCTOKIT_TEST_GITHUB_REPOSITORY ENV to set test repo name.

Default OCTOKIT_TEST_GITHUB_REPOSITORY to api-sandbox.

Add VCR filter for repository name.

Add test_github_repository helper method that points to the ENV
variable.
Add global @test_repo helper variable

Tests can reference @test_repo instead of hardcoded repository strings.
Statuses Spec: Replace hardcoded repo name string with @test_repo
CommitCommentsSpec: Add commit comment context

Spec setup a commit comment for every test in the spec even though
several tests do not need it.

Added "with commit comment" context so the setup is done for only specs
that need a new commit comment and moved those tests inside the
context".
CommitCommentsSpec: Replace hardcoded repo name with @test_repo
CommitCommentsSpec: Cleanup created commit comments
CommitsSpec: Replace hardcoded repo name with @test_repo.
ContentsSpec: Replace hardcoded repo name with @test_repo
Remove vcr tag from stubbed test in downloads spec
ObjectsSpec: Replace hardcoded repo name with @test_repo
PubSubHubbubSpec: Replace hardcoded repo name with @test_repo
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.