[DOCS] Add usage for generating code coverage report

[tlfeng]

Is your feature request related to a problem? Please describe.
#850

With PRs #971 and #1214 merged, code coverage report can be generated by gradle task.
Documentations should be added for developers about how to generate the code coverage report.

Describe the solution you'd like
Update the section in "testing" guide https://github.com/opensearch-project/OpenSearch/blob/1.0.0/TESTING.md#test-coverage-analysis

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Detailed usage is simply described in #1236