Move unit test file to proper test directory

[karlaspuldaro]

Following organization guidelines from the Elyra testing documentation, front end unit tests should be located in a folder called test in the src directory of the package being tested.

What changes were proposed in this pull request?

This PR moves pipeline-hooks.spec.ts test file from src/ into src/test/ location, as per documentation.

How was this pull request tested?

Ran make test-ui-unit and checked expected tests are still running.

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