[tests] Fix ChromeDriver timeouts and restore removed Selenium tests

[Claude]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #582.

Description of Changes

Selenium tests were intermittently hanging on GitHub Actions with ReadTimeoutError when ChromeDriver became unresponsive. This particularly affected websocket tests removed in PR #581.

Root causes:

• ChromeDriver lacked explicit timeout configuration (page load, script execution, implicit waits)
• Hard-coded 3-second waits insufficient for CI resource constraints
• time.sleep() used instead of explicit waits for alert handling

Changes:

ChromeDriver timeout configuration (tests/utils.py)

• Override get_chrome_webdriver() to set explicit timeouts:
  • set_page_load_timeout(60) - prevents indefinite page load hangs
  • set_script_timeout(60) - handles async script execution
  • implicitly_wait(10) - element search timeout

Configurable timeout values (tests/config.json)

• webdriver_wait_timeout: 20 - general WebDriverWait operations
• websocket_wait_timeout: 30 - websocket connection establishment

Alert handling improvements (tests/utils.py)

# Before: hard-coded sleep
time.sleep(2)
window_alert = driver.switch_to.alert

# After: explicit wait with timeout
WebDriverWait(driver, 5).until(EC.alert_is_present())
window_alert = driver.switch_to.alert

Test updates

• test_device_monitoring_charts, test_forgot_password: use configurable timeouts
• Restored test_websocket_marker: real-time location updates via websockets (30s timeout)
• Restored test_topology_graph: network visualization with proper wait sequences
• Restored test_create_prefix_users: RADIUS batch operations with PDF generation
• Restored geo/location tests in test_console_errors

Documentation

Added tests/SELENIUM_FIXES.md with troubleshooting guide and best practices for future test development.

Screenshot

N/A - Test infrastructure changes only.