Switch over to standard TMT reporting, don't use `result:custom`

[comps]

Since TMT gained support for "subresults", a more integrated way for a test to report fine-grained results, see ie. https://tmt.readthedocs.io/en/stable/spec/results.html , we no longer have to use the hacky result:custom.

This means we can gain (back) all the TMT features like test timeout tracking that we implement ourselves by using regular result: (not specifying it in the .fmf) and reporting extra results as the tmt-report-result command, or generating the YAML bits under subresults:.

The command would be easier, but I don't think you can specify more than 1 log to it, for any given result. But that might be a reasonable tradeoff for TMT compatibility - Oculus (testing farm) doesn't seem to display files uploaded by subresults anyway, so we could just copy all logs to test data and be done.

This would also be ATEX compatible, because it tracks test timeout correctly (and even reports it as an infra error), so we wouldn't need that extra bit of runtest.py code anyway.