Enable pylint on the tests

Description

This enables pylint on the tests/ directory. It introduces a linters/bst_pylint directory that contains pylint plugins specific to BuildStream. Currently, a single linter is added, to disable two checks that have many false positives on tests due to pytest fixtures.

This also fixes all other pylint errors found.

Changes proposed in this merge request:

• Add pylint plugin to disable unused-import and redefined-outer-name on the tests/ directory.
• Enable pylint for tests
• Fix all pylint issues found

This merge request, when approved, will close: #941 (closed)

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