SqlServiceAccount: Fix unit tests that didn't mock some of the calls

[johlju]

Pull Request (PR) description

• Changes to SqlServiceAccount
  • Fix unit tests that didn't mock some of the calls. It no longer fail
    when a SQL Server installation is not present on the node running the
    unit test (issue SqlServiceAccount: Unit tests fail to run on a machine without SQL Server installed #983).

This Pull Request (PR) fixes the following issues

• Fixes SqlServiceAccount: Unit tests fail to run on a machine without SQL Server installed #983

Task list

• Added an entry under the Unreleased section of the change log in the CHANGELOG.md.
  Entry should say what was changed, and how that affects users (if applicable).
• Resource documentation added/updated in README.md.
• Resource parameter descriptions added/updated in README.md, schema.mof
  and comment-based help.
• Comment-based help added/updated.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Resource Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

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This change is 

[codecov-io]

Codecov Report

Merging #1235 into dev will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff          @@
##            dev   #1235   +/-   ##
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  Coverage    97%     97%           
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  Files        34      34           
  Lines      4171    4171           
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  Hits       4083    4083           
  Misses       88      88

[johlju]

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