[Backwards Compatibility] update instructions in TESTING.md

fix PR comments

Partially Resolved:
opensearch-project/opensearch-build#705

Signed-off-by: Anan Zhuang <[email protected]>

Author: ananzh

TESTING.md

@@ -6,17 +6,19 @@ Overview
   - [Unit tests](#unit-tests)
   - [Integration tests](#integration-tests)
   - [Functional tests](#functional-tests)
+  - [Backwards Compatibility tests](#backwards-compatibility-tests)
 - [Continuous Integration](#continuous-integration)
 - [Environment misc](#environment-misc) 
 - [Misc](#misc) 
 
 # General information
-OpenSearch Dashboards uses [Jest](https://jestjs.io/) for unit and integration tests and [Selenium](https://www.selenium.dev/) for functional tests.
+OpenSearch Dashboards uses [Jest](https://jestjs.io/) for unit and integration tests, [Selenium](https://www.selenium.dev/) for functional tests, and [Cypress](https://www.cypress.io/) for backwards compatibility tests.
 
-In general, we recommend three tiers of tests:
+In general, we recommend four tiers of tests:
 * Unit tests: Unit tests: small and modular tests that utilize mocks for external dependencies.
 * Integration tests: higher-level tests that verify interactions between systems (eg. HTTP APIs, OpenSearch API calls, calling other plugin). 
 * End-to-end tests (e2e): functional tests that verify behavior in a web browser.
+* Backwards Compatibility tests: cypress tests that verify any changes are backwards compatible with previous versions.
 
 # Requirements
 * Install the latest NodeJS, [NPM](https://www.npmjs.com/get-npm) and [Yarn](https://classic.yarnpkg.com/en/docs/install/#mac-stable)
@@ -48,7 +50,20 @@ To debug functional tests:
 Say that you would want to debug a test in CI group 1, you can run the following command in your environment:
 `node --debug-brk --inspect scripts/functional_tests.js --config test/functional/config.js --include ciGroup1 --debug`
 
-This will print of an address, to which you could open your chrome browser on your instance and navigate to `chrome://inspect/#devices` and inspect the functional test runner `scripts/functional_tests.js`.
+This will print off an address, to which you could open your chrome browser on your instance and navigate to `chrome://inspect/#devices` and inspect the functional test runner `scripts/functional_tests.js`.
+
+### Backwards Compatibility tests
+To run all the backwards compatibility tests on vanilla OpenSearch Dashboards:
+
+`yarn test:bwc -o [test path to opensearch.tar.gz] -d [test path to opensearch-dashboards.tar.gz]`
+
+To run all the backwards compatibility tests on bundled dashboards, pass the bundle parameter to the test:
+
+`yarn test:bwc -o [test path to opensearch.tar.gz] -d [test path to opensearch-dashboards.tar.gz] -b true`
+
+To run specific versions' backwards compatibility tests, pass the versions to the test:
+
+`yarn test:bwc -o [test path to opensearch.tar.gz] -d [test path to opensearch-dashboards.tar.gz] -v "[test versions]"`
 
 ### Additional checks
 Make sure you run lint checker before submitting a pull request. To run lint checker:
@@ -70,5 +85,4 @@ By default the version of OpenSearch Dashboards will pull the snapshot of the sa
 Although Jest is the standard for this project, there are a few Mocha tests that still exist. You can run these tests by running:
 `yarn test:mocha`
 
-However, these tests will eventually be migrated. Please avoid writing new Mocha tests. For further questions or to check the status please see this [issue](https://github.com/opensearch-project/OpenSearch-Dashboards/issues/215).
-
+However, these tests will eventually be migrated. Please avoid writing new Mocha tests. For further questions or to check the status please see this [issue](https://github.com/opensearch-project/OpenSearch-Dashboards/issues/215).