add test fixture for missing file

Clone of test/packages/npm-test-files , with an entrry in `files` that does not exist.

Repro steps to verify #6582 

````
cd test/packages/npm-test-missing-files
node ../../.. pack .
````

I expect an error message and npm should not produce an output .tgz file.

Instead npm produces a tgz file which does not contain all the files specified in files[]

````
$ tar tzf npm-test-missing-files-1.2.5.tgz
package/package.json
package/.npmignore
package/include4
package/sub/include
package/sub/include2
package/sub/include4
package/test.sh
````
I'd like to avoid adding any new tests to npm that can't be run on Windows. It should be able to do this pretty easily with straight Node. I can do it, if you don't want to.