Ensure cleanup executes for pack-scoped test.

Test failure was leaving an assortment of files dangling around:

    test/tap/scoped_package/cache/@scope/generic-package/90000.100001.5/package.tgz
    test/tap/scoped_package/cache/@scope/generic-package/90000.100001.5/package/package.json
    test/tap/scoped_package/package.json
    test/tap/scoped_package/scope-generic-package-90000.100001.5.tgz