More uniform treatment of ctx.values arguments.

[bcsgh]

1. helm_package.values is a singular label type argument where as helm_lint.values and helm_template.values are lable_list.
2. [fixed] helm_template_test doesn't have a values argument at all.

Fixing case 1 would be a breaking change (thought a trivial one to for users to fix). The current work around for not being able to pass/merge multiple values files is to do that via install_opts/upgrade_opts which is kind of clunky

Case 2 can be addressed by little more than copy/pasting a few bits of _helm_template_impl into _helm_template_test_impl (I'd put together a PR for that but; "red tape").