Add a streamlined syntax for TEST_INPUT lines (#1768)

This change allows the `TEST_INPUT` syntax used by `render-test` to support aggregate values with a single input line more easily.
The test writer can now use a syntax like:

```
//TEST_INPUT:set someVar = 3.0
```

Input lines that start with the `set` keyword will now use a simpler `dst = src` format (instead of `dst:name=src` as the existing syntax used). The right-hand side expression can include:

* Numeric literals, both integer and floating-point (currently only supporting 32-bit scalar types; we could fix this later)
* Arrays, consisting of zero or more comma-separated expressions inside `[]`
* Aggregates, consisting of zero or more comma-separated "fields" inside `{}`. A field can either be `name: <expr>` or just `<expr>`
* Objects, which can be written as either `new SomeType{ <fields> }` or `new{ <fields> }` in the case where the type is know-able from context

With this approach is should be possible to support almost arbitrary-type inputs on a single line. For now, I have used this support to re-enable an existing test that had been disabled due to lack of support for setting up arrays of objects.

Major things left to do:

* The new syntax doesn't support the existing cases we had for `Texture2D`, etc. Those should probably be supported but I'd like to find a way to do it without duplicating the parsing logic (ideally the value cases from the existing code should Just Work in the new model)

* There is no support right now for non-32-bit scalar types

* It would be good if this support (and the shader cursor system) supported treating vectors like aggregates

* The actual value-setting logic doesn't currently handle aggregates without field names, so `{ a:0, b:1 }` will work but `{ 0, 1 }` will parse but fail when it comes time to set values

* While this approach lets complicated values be set with a single line, that isn't always what a user will want to do: in the future we should provide a way to break up an aggregate value over multiple lines that is consistent with this approach

* Once we port all of the relvant tests over, it would be great to drop the `set` prefix and have these lines look as simple and conventional as possible

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