Move CLI component cache build to occur before pre-processing

[kiersten-stokes]

Fixes #2911

What changes were proposed in this pull request?

This PR moves the build of the component cache for relevant CLI tasks (validate, export and submit) to occur before pipeline pre-processing. Pipeline CLI tests have also been updated to catch this case in the future. Long-winded explanation below.

Recall that ComponentCache instances are either considered server processes (usually a running Elyra instance) or not (mostly CLI processes), and only in the former case is a cache built. For the pipeline CLI tasks of validate, export and submit, we pass emulate_server_app=True to the creation/retrieval of the cache instance so that it builds and can reference the cache during pipeline validation.

The issue referenced above was caused by the fact that, as of #2799, pipeline pre-processing (e.g. the creation of a PipelineDefinition object) also creates/retrieves a ComponentCache instance (and does not specify anything in regards to emulate_server_app). Because pre-processing previously occurred before _build_component_cache in the CLI app, the ComponentCache instance was created during pre-process and did not appropriately set emulate_server_app to True. This PR moves the build of the cache to occur before pre-processing, such that creation of the ComponentCache instance is done "as a server process".

Another issue is that this problem was not caught by the tests because the component_cache fixture explicitly sets emulate_server_app to True during ComponentCache instance creation. To more accurately reflect how things flow during use of the CLI, I've created a new fixture that does not result in the creation of a ComponentCache instance.

How was this pull request tested?

Tested manually. Also ran the updated backend tests against current main to ensure that the expected tests fail on main, but pass on this branch.

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[ptitzler]

LGTM. Thank you!