Remove a bad assert and untag some tests it was affecting

Okay, here's the spiritual successor to the aborted #757.  Travis is [happy](https://travis-ci.org/dmarcotte/jruby/builds/7447511)!

I tracked down the difference between local runs and Travis runs to assertions being enabled on Travis.  A bad assert was making enemies of a bunch of working specs.

I've organized this pull into two commits to make it easier to read: the [actual code change](https://github.com/dmarcotte/jruby/commit/e55e54dbc83dd455f4b2472b585a21f837136cad) and the [subsequent test untagging](https://github.com/dmarcotte/jruby/commit/7a874d21e56bb41832af5d708be8f7accaf7f2a6).  Note that there may be specs outside of `command_line` and `core/argf` that are also brought back to life by this; I'll have a look soon.