minor refactoring of a test in `de_br.rs`

[arvidn]

Moving the helper function into the scope of the test that uses it clears up the file-scope. In a future patch I intend to introduce a similar kind of test that otherwise also would want to claim the name deserialize_check

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build 7870961238

Details

• 0 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 94.363%

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Totals
Change from base Build 7850857111:	0.002%
Covered Lines:	5758
Relevant Lines:	6102

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💛 - Coveralls