[qa] Added frontend tests #02

[emohk]

Added frontend tests for the flat JSON widget in Django admin using openwisp_utils.tests.SeleniumTestMixin. The following 5 unit tests are included:

• Add row test: Simulates adding a row using the “add” button. Verifies that a new row appears and that it is correctly saved to the database after clicking save button.
• Remove row test: Creates a sample test json document with 3 rows. Removes the second row and verifies that only 2 rows remain, while the first and third rows remain the same.
• Toggle text area test: Verifies that clicking the “toggle textarea” button hides the form rows and add-row button, and displays the raw JSON textarea instead.
• Editting json from textarea test: Creates a sample test json document with 3 rows. Switches to textarea mode, removes one row by editing the raw JSON, and verifies that the changes are saved correctly.
• Widget mode and raw mode sync test: Modifies JSON in the textarea, toggles back to widget mode without saving, and checks that the changes are visible in the form row of widget mode.

Fixes #2