Refactor devfs_web_ddc_modules_test.dart (#155609)

This pull request adds a local function `runInTestbed()` to **devfs_web_ddc_modules_test.dart**, which wraps the `testbed.run()` method. Several whitespace adjustments have been made as well.

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It's much easier to read after clicking "hide whitespace".

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Relevant style guidelines:
- Prefer avoiding line breaks after assignment operators.
- If you have a newline after some opening punctuation, match it on the closing punctuation.
- Only use `=>` when everything, including the function declaration, fits on a single line.

Author: nate-thegrate