More complete unit tests for Azure Blob Storage backend (#1138)

* More complete unit tests for Azure Blob Storage backend

Pull request #1109 introduced key prefix support for the Azure backend, with
unit tests for `normalize_key()`, but those tests didn't test whether the
Storage impl *actually* used `normalize_key()` when constructing an API
request to Azure.

This change makes the unit tests more complete, by testing the Storage impl
directly through the use of a mock HTTP server as Azure API endpoint.

* Split up Azure storage tests

Author: FooBarWidget

Cargo.lock

@@ -98,6 +98,7 @@ predicates = "=2.1.1"
 thirtyfour_sync = "0.27"
 once_cell = "1.9"
 serial_test = "0.5"
+wiremock = "0.4.9"
 
 [target.'cfg(unix)'.dependencies]
 daemonize = "0.4"