chore: Enable per-branch test suite execution in CircleCI

[jimblanc]

Description of changes

This change enables us to execute the CircleCI test suite against corresponding branches in the amplify-js-samples-staging repo. Currently this implementation requires an exact match. If no match is found, then CircleCI will default to main.

Why couldn't we just use $? instead of setting a local variable? This was required because if any command in the list exits with a failure status code, CircleCI will fail the test run.

Issue #, if available

Description of how you validated changes

Test runs in CircleCI with and without a matching branch.

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.

[iartemiev]

LGTM 👍🏻

[codecov-commenter]

Codecov Report

Merging #10702 (7a5270e) into main (2320dba) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #10702      +/-   ##
==========================================
+ Coverage   85.64%   85.65%   +0.01%     
==========================================
  Files         196      196              
  Lines       18261    18261              
  Branches     3892     3892              
==========================================
+ Hits        15640    15642       +2     
+ Misses       2545     2543       -2     
  Partials       76       76              

Impacted Files	Coverage Δ
...kages/amazon-cognito-identity-js/src/BigInteger.js	89.71% <0.00%> (+0.40%)	⬆️

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