fix(cli): SynthUtils is not used

[richardhboyd]

unused imports makes the jsii fail because jsii has no chill.

.test.ts:1:45 - error TS6133: 'SynthUtils' is declared but its value is never read.
1 import { expect as expectCDK, haveResource, SynthUtils } from '@aws-cdk/assert';
                                              ~~~~~~~~~~

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildProject6AEA49D1-qxepHUsryhcu
• Commit ID: 6108163
• Result: SUCCEEDED
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Thank you for contributing! Your pull request will be updated from master and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).