[#6292] Add unit tests for AzureBlobStorage

[ceciliaavila]

Addresses #6292
#minor

Description

This PR removes the existing Functional tests for the AzureBlobStorage class and replaces them with unit tests that don't require the use of the Azure Storage Emulator.

Specific Changes

• Updates the AzureBlobStorage class, adding a new internal constructor that receives an instance of CloudBlobClient. This constructor is for the use of the tests only.
• Replaces the existing tests in the AzureBlobStorageTests class with unit tests for each method, constructor, method validations, etc, reaching 89% of code coverage.

Testing

The following images show the new unit tests and the code coverage.

[coveralls]

Pull Request Test Coverage Report for Build 304856

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 25 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 78.945%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Dialogs.Adaptive/Actions/ForEachElement.cs	8	90.0%
/libraries/Microsoft.Bot.Builder/BotFrameworkAdapter.cs	17	47.91%

Totals
Change from base Build 301524:	0.2%
Covered Lines:	25373
Relevant Lines:	32140

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💛 - Coveralls

[BruceHaley]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll