Add test repository setup and teardown helpers.

Added `test_repo` test helper method to generate a unique-enough
repository name and filter it from VCR cassettes as
`<GITHUB_TEST_REPOSITORY>`.

Added `setup_test_repo` test helper method to create a new repository on
GitHub to test against. Method returns the newly created repository.

Added `teardown_test_repo` test helper method to delete a test repository
from GitHub. Method takes a full repository name.

These methods will remove the need for hardcoded test repository names which cause headaches for anyone working on existing tests or adding new methods instead of a group of tests that utilize any type of setup in a `before` block. Example usage can be seen in https://github.com/octokit/octokit.rb/pull/403#issue-25510917.