Failing test for IO-890

[sparkhi]

This PR shows the failing test only.
The test uses java.io.File to get the filename, same can be used in the implementation, however it interferes with other expectations of getName and getBaseName hence I've not proposed implementation changes.

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[garydgregory]

@sparkhi
Please fix:

[INFO] --- checkstyle:3.6.0:check (default-cli) @ commons-io ---
[INFO] There are 4 errors reported by Checkstyle 13.4.0 with D:\a\commons-io\commons-io/src/conf/checkstyle.xml ruleset.
Error:  src\test\java\org\apache\commons\io\FilenameUtilsTest.java:[1211] (regexp) Regexp: Line matches the illegal pattern 'Trailing whitespace'.
Error:  src\test\java\org\apache\commons\io\FilenameUtilsTest.java:[1215,18] (coding) FinalLocalVariable: Variable 'testFileWithBackSlash' should be declared final.
Error:  src\test\java\org\apache\commons\io\FilenameUtilsTest.java:[1216,20] (coding) FinalLocalVariable: Variable 'actualFileName' should be declared final.
Error:  src\test\java\org\apache\commons\io\FilenameUtilsTest.java:[1217,20] (coding) FinalLocalVariable: Variable 'expectedFileName' should be declared final.