Fix modified logger in add_file_handler, add tests

[cottsay]

It appears that the logger parameter was previously unused, and that the function just used colcon_logger directly. As it happens, the only use of add_file_handler passes colcon_logger, so the mistake had no effect.

This fixes the bug and adds a simple test for that code.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.49%. Comparing base (b174608) to head (5cdb6c0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #649 +/- ## ========================================== + Coverage 83.43% 83.49% +0.05% ========================================== Files 66 66 Lines 3816 3816 Branches 745 745 ========================================== + Hits 3184 3186 +2 + Misses 555 554 -1 + Partials 77 76 -1 ```

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