[#6292] Add unit tests to cover AzureBlobTranscriptStore

[Cecilia Avila (ceciliaavila)]

Addresses #6292
#minor

Description

This PR adds unit tests for the AzureBlobTranscriptStore class.

Specific Changes

• Adds unit tests for the AzureBlobTranscriptStore class.
• Adds a new internal constructor that receives a CloudBlobClient to be able to pass a mocked instance of this class, allowing to test without the use of emulators.

Testing

This image shows the unit tests passing and the code coverage.

[Coveralls (coveralls)]

Pull Request Test Coverage Report for Build 304880

• 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 78.948%

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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 302179:	0.2%
Covered Lines:	25374
Relevant Lines:	32140

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💛 - Coveralls

[BruceHaley]

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