Fix test artifact output location bugs

[ptitzler]

Fixes https://github.com/elyra-ai/elyra/issues/3116. The root cause are two KFP processor tests, which produce the YAML file artifacts due to bugs:

$ pytest elyra/tests/pipeline/kfp/test_processor_kfp.py -k test_generate_pipeline_dsl_compile_pipeline_dsl_one_generic_node_pipeline_test_2

$ pytest elyra/tests/pipeline/kfp/test_processor_kfp.py -k test_generate_pipeline_dsl_compile_pipeline_dsl_generic_components_data_exchange

What changes were proposed in this pull request?

Strip absolute path information from generated file names

How was this pull request tested?

• pytest elyra/tests/pipeline/kfp/test_processor_kfp.py passes all tests and git status does not list any new artifacts

$ pytest -v elyra/tests/pipeline
 ...
======================================== 222 passed, 8 warnings in 55.19s ========================================
$ git status
On branch code-gen-test-artifacts
nothing to commit, working tree clean

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