Update tests ESM references (joomla#44559)

Signed-off-by: Emmanuel Ferdman <[email protected]>

Author: emmanuel-ferdman

tests/System/README.md

@@ -197,7 +197,7 @@ The Database Commands create items in the database like articles or users. They
 cy.db_createArticle({ title: 'automated test article' }).then((id) => { ... })`
 ```
 
-The following commands are available and are served by the file [tests/System/support/commands/db.js](/tests/System/support/commands/db.js):
+The following commands are available and are served by the file [tests/System/support/commands/db.mjs](/tests/System/support/commands/db.mjs):
 
 - **db_createArticle** – Creates an article and returns the id
 - **db_createBanner** – Creates a banner and returns the id
@@ -230,7 +230,7 @@ cy.api_get('/content/articles').then((response) => { ... })`
 ```
 The response is an object from the [Cypress request command](https://docs.cypress.io/api/commands/request).
 The following commands are available and are served by the file
-[tests/System/support/commands/api.js](/tests/System/support/commands/api.js):
+[tests/System/support/commands/api.mjs](/tests/System/support/commands/api.mjs):
 
 - **api_get** – HTTP GET request for given path
 - **api_post** – HTTP POST request for given path and body