Unit tests for vkjson_gen_util (Part 2/3)

* This CL adds unit tests for all the 3 utility functions involved in the generation of vkjson.h with 100% coverage
* Slightly modifies one of the existing test class to take advantage of the newly introduced "BaseMockCodeFileTest"

Reference:
Part 1/3 CL - https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/native/+/33070222

Bug: b/409449379
Flag: NONE infeasible
Test: No impact to vkjson src files. All 53 test cases pass when executed locally

Change-Id: Ie599e28c9d39c667887ded645ec50d2c2430387c