provide wiki doc that explains process for testing against any endpoint

[anodelman]

The versitygw bats tests should be executable against other endpoints (eg, s3). We need to provide doc coverage of how to conduct that testing. Since the tests do include some non-standard test coverage (where versitygw specifically differs from spec) these tests should be easy to exclude/skip.

Full automation of these tests may not be necessary, but should still be run periodically.