Closes#1346
`GetObject` and `HeadObject` return the `x-amz-tagging-count` header in the response, which specifies the number of tags associated with the object. This was already supported for `GetObject`, but missing for `HeadObject`. This implementation adds support for `HeadObject` in `azure` and `posix` and updates the integration tests to cover this functionality for `GetObject`.
All the integration tests used to be in a single file, which had become large, messy, and difficult to maintain. These changes split `tests.go` into multiple files, organized by logical test groups.
