Pass context to test when using select

[rgroothuijsen]

This bugfix allows pass_context to be used by tests in combination with select, which was previously failing because select did not pass the context object, and thus the context would remain None.

Fixes #1624.

Regarding the request for updating CHANGES.rst: should I add a new version number (i.e. 3.1.3) to the document for this bugfix? I'm assuming that 3.1.x is the correct target branch for this PR, but I wasn't entirely sure.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.