test(cli): add test for get-plugin-instance extensions

[fossamagna]

Description of changes

Increase test coverage of amplify-cli/extensions.

Description of how you validated changes

lerna --scope @aws-amplify/cli run test

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #7233 (65ab04c) into master (0b20951) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7233      +/-   ##
==========================================
+ Coverage   52.27%   52.28%   +0.01%     
==========================================
  Files         491      492       +1     
  Lines       25248    25254       +6     
  Branches     4952     4954       +2     
==========================================
+ Hits        13199    13205       +6     
  Misses      11097    11097              
  Partials      952      952              

Impacted Files	Coverage Δ
.../extensions/amplify-helpers/get-plugin-instance.ts	100.00% <0.00%> (ø)

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[cjihrig]

Thank you for the contribution.