refactor(sam): remove duplication in sam test file

[Hweinstock]

Problem

This is the second longest case of code duplication in the repo from jscpd (38 lines!).

Solution

Provide helper methods to build test objects.

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License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[justinmk3]

Please check this file on the https://github.com/aws/aws-toolkit-vscode-staging/tree/feature/lambda-get-started branch. If it still looks mostly the same there, then we can merge this. If it's removed there or heavily modified, we might want to skip this.

[Hweinstock]

Looks like we're good to go based on file commit history.
Staging: https://github.com/aws/aws-toolkit-vscode-staging/commits/feature/lambda-get-started/packages/core/src/test/lambda/config/samParameterCompletionItemProvider.test.ts
Here: https://github.com/aws/aws-toolkit-vscode/commits/5fa03f4b027d7bf8dfc7791ac03d4df9ba0dee76/packages/core/src/test/lambda/config/samParameterCompletionItemProvider.test.ts