Fix mypy reexport error for testfixtures

[nsoranzo]

Fix this mypy error for the galaxy-app package:

tests/app/jobs/dynamic_tool_destination/test_dynamic_tool_destination.py:5: error:
Module "testfixtures" does not explicitly export attribute "log_capture";
implicit reexport disabled  [attr-defined]
    from testfixtures import log_capture
    ^

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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