Expose basic api for testing external plugins.

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

Moves the following files from tests/testutils/ to
buildstream/plugintestingutils/:
  * runcli.py
  * integration.py