SqlDatabaseRole: Add a test to check error message on helper function

[jpomfret]

Pull Request (PR) description

Adds a test to ensure Add-SqlDscDatabaseRoleMember throws the expected error.

This Pull Request (PR) fixes the following issues

• Fixes SqlDatabaseRole: Missing unit test for helper function Add-SqlDscDatabaseRoleMember #1620

Task list

• Added an entry to the change log under the Unreleased section of the
  file CHANGELOG.md. Entry should say what was changed and how that
  affects users (if applicable), and reference the issue being resolved
  (if applicable).
• Resource documentation updated in the resource's README.md.
• Resource parameter descriptions updated in schema.mof.
• Comment-based help updated, including parameter descriptions.
• Localization strings updated.
• Examples updated.
• Unit tests updated. See DSC Community Testing Guidelines.
• Integration tests updated (where possible). See DSC Community Testing Guidelines.
• Code changes adheres to DSC Community Style Guidelines.

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This change is 

[codecov]

Codecov Report

Merging #1626 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1626   +/-   ##
======================================
  Coverage      98%     98%           
======================================
  Files          37      37           
  Lines        6014    6014           
======================================
  Hits         5925    5925           
  Misses         89      89           

Flag	Coverage Δ
#unit	98% <ø> (ø)

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[johlju]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[johlju]

Sorry it took so long for me to review this. Work is extreme at the moment. 🙂

[jpomfret]

No problem at all. Thanks!