AttachmentIT UI test cannot run on NTFS filesystems

One of the tests in AttachmentIT, `checkEscapingInAttachmentName`, checks that filenames are properly escaped and are not interpreted as HTML during the upload. The current implementation of this test creates a local file `<strong>EscapedAttachment.txt` and uploads it on the tested instance.

Such a filename is invalid on NTFS filesystems, which means that the test will surely failed when executed on Windows. We need a better way to send resources to the test container that do not rely on creating the file locally first.

See https://github.com/xwiki/xwiki-platform/pull/3524