Add two new test cases, TestBackendAzureAccountToken and
TestBackendAzureContainerToken, that ensure that the authorization using
both types of token works.
This introduces two new environment variables,
RESTIC_TEST_AZURE_ACCOUNT_SAS and RESTIC_TEST_AZURE_CONTAINER_SAS, that
contain the tokens to use when testing restic. If an environment
variable is missing, the related test is skipped.