Fix setting linked datasources for reports #398

[danubie]

Fixes #398

Changes proposed in this pull request:

• check datasource property IsReference
• correct test context message

How to test this code:

• Pester test added
• Test will fail with current master branch version, succeed with this PR

Has been tested on (remove any that don't apply):

• Powershell 5.1 and 7.2.11
• Windows 10
• SQL Server 2017

[danubie]

@microsoft-github-policy-service agree company="Inhouse GesmbH"