A new storage account is created each time a test runs.  It makes each test take longer.
Also, this creates unnecessarily many storage accounts.  (For example, one test run creates two storage account, one in AssemblyInitialize and another in TestInitialize)
Without proper cleaning up, it could end up test failures if the number of the storage accounts the test created reaches a limit.
I think, it is better not to create a new storage account if the test run already created a storage account.