[fix] Fixed the default test #139

[pandafy]

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 #139

Description of Changes

• Increased test_retries to 10
• If the default test fails, the agent will wait randomly between 5 and 20 seconds before re-attempting the test.
• Improved logging for failures.

[pandafy]

I have tested the scenario where the server returns 502 Bad Gateway response for default test on OpenWrt 24.10

Mon Feb 17 15:17:43 2025 daemon.info openwisp: Testing configuration...
Mon Feb 17 15:17:43 2025 daemon.warn openwisp: Configuration test passed, but the controller returned HTTP/1.1 502 Bad Gateway
Mon Feb 17 15:17:43 2025 daemon.info openwisp: Configuration test succeeded
Mon Feb 17 15:17:43 2025 daemon.info openwisp: Configuration applied successfully

I have also tested the scenario when the default test fails when the connection timed out. The agent retried for 10 times

Mon Feb 17 15:19:36 2025 daemon.info openwisp: Local configuration outdated
Mon Feb 17 15:19:36 2025 daemon.info openwisp: Downloading configuration from controller...
Mon Feb 17 15:19:36 2025 daemon.info openwisp: Configuration downloaded, now applying it...
Mon Feb 17 15:19:37 2025 daemon.info openwisp: Service system has been reloaded via procd/ubus
Mon Feb 17 15:19:42 2025 daemon.info openwisp: Testing configuration...
Mon Feb 17 15:20:12 2025 daemon.warn openwisp: Configuration test failed (attempt 1). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:20:51 2025 daemon.warn openwisp: Configuration test failed (attempt 2). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:21:33 2025 daemon.warn openwisp: Configuration test failed (attempt 3). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:22:09 2025 daemon.warn openwisp: Configuration test failed (attempt 4). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:22:54 2025 daemon.warn openwisp: Configuration test failed (attempt 5). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:23:41 2025 daemon.warn openwisp: Configuration test failed (attempt 6). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:24:18 2025 daemon.warn openwisp: Configuration test failed (attempt 7). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:25:03 2025 daemon.warn openwisp: Configuration test failed (attempt 8). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:25:41 2025 daemon.warn openwisp: Configuration test failed (attempt 9). Failed to connect to controller: curl exit code 28
Mon Feb 17 15:26:19 2025 daemon.warn openwisp: Configuration test failed (attempt 10). Failed to connect to controller: curl exit code 28