Fix interface_check failure in test_cont_warm_reboot

[vaibhavhd]

Description of PR

Summary: Fix test_cont_warm_reboot that has started failing after the recent restructuring done in https://github.com/Azure/sonic-mgmt/pull/2883/files#

Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Approach

test_cont_warm_reboot is failing with:

=================================== FAILURES ===================================
____________________________ test_continuous_reboot ____________________________
Fixture "check_interfaces" called directly. Fixtures are not meant to be called directly,
but are created automatically when test functions request them as parameters.

This error is due to recent refactoring which made check_interfaces a fixture (previously a method).

What is the motivation for this PR?

How did you do it?

Added changes to access fixture on demand, and perform the interface status validation.

How did you verify/test it?

Tested on a KVM switch and the test passed. With interface made manually down, following error is gracefully generated avoiding a test crash:

{
    "up_time": "2043.0s", 
    "test_duration": "93.658068s", 
    "image": "SONiC-OS-202012.15-03b26d43", 
    "result": false, 
    "is_new_image": false, 
    "test_id": 1, 
    "checks": {
        "check_interfaces_and_transceivers": "Interface check failed, not all interfaces are up. Failed: [{'check_item': 'interfaces', 'failed': True, 'host': 'vlab-01', 'down_ports': [u'Ethernet4']}]", 
        "verify_no_coredumps": true, 
        "check_reboot_type": true, 
        "verify_image": true, 
        "check_neighbors": true, 
        "check_services": true
    }

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation