[tests] Retry failing selenium tests

[nemesifier]

We need a mechanism which would automatically retry failing selenium tests a couple of times before flagging them as failed.

With the help of ChatGPT I generated a prototype for this:

import unittest
import time
import traceback
import sys


class SeleniumTestMixin:
    retry_max = 2
    retry_delay = 0  # seconds

    def run(self, result=None):
        """
        Override unittest.TestCase.run to retry flaky tests.
        """
        original_result = result
        test_name = self.id()

        for attempt in range(self.retry_max + 1):
            temp_result = self.defaultTestResult()
            super().run(temp_result)

            if temp_result.wasSuccessful():
                if original_result is not None:
                    original_result.addSuccess(self)
                return

            if attempt < self.retry_max:
                print("-" * 80)
                print(
                    f"[Retry] Test '{test_name}' failed on attempt {attempt + 1}. Retrying..."
                )
                print("-" * 80)
                if self.retry_delay:
                    time.sleep(self.retry_delay)
                continue
            else:
                for failed_test, err in temp_result.failures + temp_result.errors:
                    print("-" * 80)
                    print(f"[Traceback for '{failed_test.id()}']:\n")
                    print(err.strip())
                    print("-" * 80)

                if original_result is not None:
                    for failure in temp_result.failures:
                        original_result.addFailure(*failure)
                    for error in temp_result.errors:
                        original_result.addError(*error)

We need to update the docs to state that selenium tests are automatically retried and document retry_max and retry_delay, plus do some manual testing to ensure that the messages printed when flaky tests are retried are visible and readable.