Replaced deprecated sinon method in test:unit

[mattc41190]

Proposed changes

When running the tests I noticed that Sinon was warning about:

sinonjs/sinon#1630

So I switched it

Types of changes

• [ x] Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x ] I have read the CONTRIBUTING doc
• [x ] I have added tests that prove my fix is effective or that my feature works
• [x ] I have added necessary documentation (if appropriate)

Further comments

Reviewers: @christian-bromann

[jsf-clabot]

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[mattc41190]

work user -- switching user