kunit: tool: remove redundant file.close() call in unit test

We're using a `with` block above, so the file object is already closed.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>