Use a file object to generate a our test file instead of calling `dd`;
removes dd's output from testimage.log, keeps unittest output clean.
Also remove unused imports.
(From OE-Core rev: 6ac48ffbab29a37b0eada533191878aeae3c91f0)
Signed-off-by: Mihai Lindner <mihaix.lindner@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>