TrigValidation: Move PhysValWebStep from CheckSteps to TrigAnalysisSteps, improve configuration and error reporting

Move the web display generation step from TrigValTools.TrigValSteering.CheckSteps to TrigAnalysisTest.TrigAnalysisSteps, as it is only used in TrigAnalysisTest and fits better in there. At the same time, change its logic a bit to make it more robust against failures and improve the error reporting to detect the underlying issues like discussed in ATR-21841. A helper function is also added to add a set of these steps to a test, and the three relevant test files are simplified.

The "compLegacy" test is also changed to compare Run-3 and Run-2 trigger outputs both from the previous nightly instead of one from the current and one from the previous nightly.