Fix test artifact output location bugs

[ptitzler]

Fixes #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

Developer's Certificate of Origin 1.1

   By making a contribution to this project, I certify that:

   (a) The contribution was created in whole or in part by me and I
       have the right to submit it under the Apache License 2.0; or

   (b) The contribution is based upon previous work that, to the best
       of my knowledge, is covered under an appropriate open source
       license and I have the right under that license to submit that
       work with modifications, whether created in whole or in part
       by me, under the same open source license (unless I am
       permitted to submit under a different license), as indicated
       in the file; or

   (c) The contribution was provided directly to me by some other
       person who certified (a), (b) or (c) and I have not modified
       it.

   (d) I understand and agree that this project and the contribution
       are public and that a record of the contribution (including all
       personal information I submit with it, including my sign-off) is
       maintained indefinitely and may be redistributed consistent with
       this project or the open source license(s) involved.

[kevin-bates]

Looks good @ptitzler - thank you!