Support E2E stress tests in run_tests.js (#29682)

* [skip ci] Adapt run_tests.js for stress testing
* [skip ci] Make channel_read_after_permalink_spec.js retry-able
* [skip ci] Fix ui_commands.ts fix
* [skip ci] Improve postMessageAndWait
* [skip ci] Fix unread_on_public_channel_spec.js
* [skip ci] Demote unread_on_public_channel_spec.js
* Update e2e-tests/README.md

Co-authored-by: Saturnino Abril <5334504+saturninoabril@users.noreply.github.com>

* [skip ci] Fix mesage_reply_bot_post_spec.js

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Saturnino Abril <5334504+saturninoabril@users.noreply.github.com>

e2e-tests/README.md

@@ -61,3 +61,14 @@ Notes:
 
 ##### For code changes:
 * `make fmt-ci` to format and check yaml files and shell scripts.
+
+##### For test stressing an E2E testcase
+
+For Cypress:
+1. Enter the `cypress/` subdirectory
+2. Identify which test files you want to run, and how many times each. For instance: suppose you want to run `create_a_team_spec.js` and `demoted_user_spec.js` (which you can locate with the `find` command, under `cypress/tests/`), each run 3 times
+3. Run the chosen testcases the desired amount of times: `node run_tests.js --include-file=create_a_team_spec.js,demoted_user_spec.js --invert --stress-test-count=3`
+  * Your system needs to be setup for Cypress usage, to be able to run this command. Refer to the [E2E testing developer documentation](https://developers.mattermost.com/contribute/more-info/webapp/e2e-testing/) for this.
+4. The `cypress/results/testPasses.json` file will count, for each of the testfiles, how many times it was run, and how many times each of the testcases contained in it passed. If the attempts and passes numbers do not match, that specific testcase may be flaky.
+
+For Playwright: WIP