Fixed missing callings of `fclose()` in the unit tests

[fktn-k]

I've noticed that the test with Valgrind emits warnings about possible memory leaks.
Although it's turned out missing callings of the std::fclose() function in the unit tests for encode detector is the cause and that doesn't affect the fkYAML library itself, it should be better to fix them.
No other fixes are included in this PR.

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• [x] Changes are described in the pull request or in a referenced issue.
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[coveralls]

coverage: 100.0%. remained the same when pulling 719a553d4d4f2a773336a3c24a8599c1a76ffbcd on bugfix/fix_valgrind_memory_leaks_warnings into e98d1d0fee114c76a998f5f1191bd9f6997bfc95 on develop.