Added FileFolderExtensionTests.cs class and other Unit Tests for extensions

1) Added FileFolderExtensionTests.cs class and tests for CheckIn and CheckOut files
2) Added Office365.png file in the Resources folder for tests against files.
3) Re-factored TestCommon.cs so that Context Creation logic is locates in one function.
4) Added CreateList Test in ListExtensionTests.cs