simplify handlers test

[ptitzler]

Simplify code maintenance by replacing a test resource with a link to the original resource, which is modified on the fly to produce the required input. This reduces the efforts required whenever a generic pipeline property needs to be added, changed, or removed. This also avoids the problem that tests might utilize a resource that is not a true copy of the original. An example of such a discrepancy (property order is inconsistent) can already be observed: https://github.com/elyra-ai/elyra/blob/main/elyra/templates/components/generic_properties_template.jinja2#L16 vs https://github.com/elyra-ai/elyra/blob/main/elyra/tests/pipeline/resources/properties.json#L9

What changes were proposed in this pull request?

• See above.
• I also considered not creating a symlink to the original resource (and loading the original resource instead) but decided against it. The main motivation is that the chosen approach is a bit more explicit and provides a hint that a test is using a resource that is not located in the elyra/tests directory.

How was this pull request tested?

pytest -v elyra/tests/pipeline/test_handlers.py

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[kiersten-stokes]

I was just thinking about this again the other day! Very satisfying 😄