Fix VCR repository name string replacement

Using api-sandbox as the default as well as api-sandbox-{Time} pattern
for the unique name causes the recording to save the incorrect partial
match.

"<GITHUB_TEST_REPOSITORY>-1389693666.0936031"

After changing unique to octokit-test-repo-{Time} pattern the string is
replaced completely as intended.

"<GITHUB_TEST_REPOSITORY>"
Change basic_auth_client test helper to create new client each time.

This causes sporatic test failures when an existing client isn't reset
and is already authed with Oauth, causing the client returned by
basic_auth_client to actually be an oauth_client therefore causing VCR
requests to not match the basic auth URI. Since the tests are run in
random order to help isolate them, it also helps this fun functionality
to be random and only fail sometimes, usually on the CI so the world can
laugh at you for making the tests fail.