Improve unit test system with additional test case functionality

[igorpecovnik]

Description

• test script can have function testcase which includes a script that needs to be executed
• CONDITION and PREINSTALL variables are deprecated as general condition is that function testcase completes
• unit tests are now run on latest Armbian images

Implementation Details

Test config example:

ENABLED=true
RELEASE="bookworm:noble"

function testcase {
        ./bin/armbian-config --api module_cockpit install
        [ -f /usr/bin/cockpit-bridge ]
}

Documentation Summary

Updated Readme.md in the tests folder.

Testing Procedure

• All test cases has been moved to new system

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have ensured that my changes do not introduce new warnings or errors
• No new external dependencies are included
• Changes have been tested and verified
• I have included necessary metadata in the code, including associative arrays

[Tearran]

This is great.