Expose basic api for testing external plugins.

Description

Currently external plugins are maintaining their own copies of the testing utilities they need.

We want external plugins to be able to make use of the core testing utils.

This MR exposes the basic utilities which are currently in use in bst-external plugins. If necessary, more utilities could be exposed in the future.

Changes proposed in this merge request: Make the utilities from the following files from tests/testutils/ publicly available in the module buildstream.plugintestingutils:

• runcli.py
• integration.py

This merge request, when approved, will close: #847 (closed)

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