(fix) remove export of createManagerTest from index

[potench]

Fixes #29

Change Log
(fix) remove an unnecessary import of createManagerTest.js in index.js - fixes unnecessary bundling of mockGPT and createManagerTest in production bundle.

Test Plan

1. npm run build
2. Review dist/react-gpt.js to ensure createManagerTest and mockGPT are not imported.
3. npm run test run tests to make sure everything still works

[codecov]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #68   +/-   ##
=======================================
  Coverage   92.93%   92.93%           
=======================================
  Files           8        8           
  Lines         580      580           
=======================================
  Hits          539      539           
  Misses         41       41

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[potench]

After this change:
bundlesize — ./dist/react-gpt.min.js: 8.39kB < maxSize 10.5kB gzip
Updated bundlesize to reflect change