Deprecate test syntax that overrides `RegressionTest` methods

[vkarak]

[ekouts]

@vkarak Do we want the deprecation warning only when the user overwrites a function from the pipeline, like run or setup, or when they try to overwrite any function of the RegressionTest object, like depends_on, __str__ etc?

[vkarak]

Yes, I'd say so, but we would need another decorator to decorate a subclass that it actually extends the functionality. We don't want to ban overriding RegressionTests functionality, but we want to discourage it for normal tests.

[ekouts]

In the PR for this issue or as a separate feature?

[vkarak]

No, in this one.