Break out query parsing from setting the query in the model...

...so FindInFilesUI doesn't need to touch the model before the search starts (for #6966).

@peterflynn This approach makes it so we don't have to deal with multiple models. There really wasn't any reason for FindInFilesUI to touch the model before actually starting the search, because all it needed was to parse the query and get error info. One the search starts, it's ok to clear the model at that point (and the existing code already does that in `FindInFiles.doSearchInScope()`).
Those functions contain their own `runs()` blocks, so they don't need them here (in fact, I think in general Jasmine says you shouldn't nest `runs()` blocks, though in practice they seem to work out fine since everything gets serialized properly anyway, as long as you don't have other non-`runs()` code between them).