expose test matrix through /testing folder

[dwarakaprasad]

fixes #25384

With this change, blueprints will have access to test/support i.e. matrix.

usage:
import { clientSamples, buildClientSamples, buildServerMatrix } from 'generator-jhipster/testing';

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[mshima]

Please move exported methods to testing.
So we have a clear distinction of libs that should be stable during major cycle.
Test folder is not stable.

[dwarakaprasad]

Majority of the changes are related to testHelpers* that are imported through test/support/index.js (which was exporting ../testing/helpers.js). I have refactored this to import from testing/index.js.

[mshima]

Thanks @dwarakaprasad