Remove duplicate testOutputsDirectory definition from integration_test package

[jakemac53]

I noticed a getter by this name with the same implementation already exists in flutter_driver, with the same implementation as the initializer for this variable. This deletes the variable and instead exports the getter.

Note that technically this could be breaking, because this version is a mutable variable instead of just a getter. But, I think this was just a mistake.

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• I added new tests to check the change I am making, or this PR is test-exempt.
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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[nate-thegrate]

This looks great!

[stuartmorgan-g]

test-exempt: code refactor with no semantic change

(Modulo the change flagged in the PR description; if we assume that's a mistake then there's no change in intended supported behavior.)