Allow the use of Windows Credential Manager instead of hardcoded values

This allows the use of a Credential Manager entry instead of specifying the username and password in the appconfig for the test project in core. It's enough to specify the SPOCredentialManagerLabel entry in the app.config with an label of an entry in the Windows Credential manager to pick up that value. This would prevent accidentally committing usernames and passwords.

Fixed a wrong check in TestCommon too (it was checking 4 times for the existence of the tenanturl)