image/kernel-tracking-stable is executed in beta testing

### Bug Description

image/kernel-tracking-stable checks if the snap is in stable channel but it doesn't make sense for testing snap in beta.
It should be removed as it produces redundant error.
Example: http://10.102.156.15:8080/job/cert-advantech-uno-2271g-v2-intel-kernel-beta/59/testReport/
Johnatan commented:
> i think this should be one of the tests that gets a manifest requirement of "in_maintenance_testing" or whatever it is proposed to call it

### To Reproduce

Launch SRU test plan for beta kernel.

### Environment

It's not environment dependent. The test shouldn't be executed for beta testing.

### Relevant log output

_No response_

### Additional context

_No response_

Start with the image/kernel-tracking-stable test entry while running the SRU test plan for a beta kernel, and inspect how tests are selected for beta testing. The work is done when this test no longer produces the redundant error for beta kernels, either by removal or by the proposed manifest requirement.

Written by the indexing model from the issue text.