Revisit spec/frontend/commit/pipelines/pipelines_table_spec.js

The spec at spec/frontend/commit/pipelines/pipelines_table_spec.js might need to be revisited a bit:

• It mounts the tested component with mount, which we might be able to switch to shallowMount to speed up the tests execution.
• It leverages a findModal helper that looks for the GlModal component. However, the fully mounted component seems to render at least 2 modals, which might make this helper unpredictable.
• It has some mobile and desktop-specific test cases, but they all seem to share the same setup and assertions, we might be able to de-duplicate things there.
• Test help paths in the modals.