fix: keep generated testcases files and assets

[jeeyyy]

This PR solves for the missing test cases html files when publishing the site. A bug was recently introduced as a part of #604, where the logic for generating the test cases was moved out of a gatsby createNode hook, into a standalone npm script namely createTestcases. This npm script was executed as a prebuild step to build > gatsby build. Although all the necessary test case files and assets were created in the public directory as expected in the script, they were being deleted by gatsby build during its preInit step as shown below. (Note: gatsby develop does not clear these files) & hence these files were missing when the site was published.

This PR fixes the above issue, but changing the npm script to generate the test cases to a temporary directory _data/rules-testcases. Here all the necessary files, their assets the testcases.json files are generated. This directory can then be used as a one source of truth for all test cases.

The contents of the directory are then copied across to the public directory during the preBootstrap hook of the gatsby build step. This ensures that the files are not cleaned up during the preInit step.

Refer https://www.gatsbyjs.org/docs/node-apis/#onPreInit for more information on the various API's.

Note: research was done to see if deleting html and css assets can be bypassed during the gatsby build step. But this was not a configurable option & is a core step for invalidating the .cache internally for gatsby

Closes issue(s):

• NA

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