Tests DOM.setFileInputFiles method.
name: file1.txt, webkitRelativePath: file-input-directory-upload/file1.txt, content: test1
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name: file2.txt, webkitRelativePath: file-input-directory-upload/file2.txt, content: test2
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name: file3.txt, webkitRelativePath: file-input-directory-upload/subdir/file3.txt, content: test3
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