Added FileFolderExtensionsTests.cs class and other Unit Tests for extensions

1) Added FileFolderExtensionsTests.cs class and tests for CheckIn and CheckOut files
2) Added Office365.png file in the Resources folder to use during File Unit Tests.
3) Re-factored TestCommon.cs so that Context Creation logic is located in one function.
4) Added CreateList Test in ListExtensionTests.cs