Add quirks test for unanchored filter args

@fw42 

Added a quirks test for the unanchored parsing of filter arguments. This caused problems with `liquid/c` in Shopify/shopify#25531

This behaviour is super messed up and stems from the regex `scan` which is not anchored to match things that are adjacent and starting from the beginning of the string.
```ruby
# Variable#lax_parse
filterargs = f.scan(/(?:#{FilterArgumentSeparator}|#{ArgumentSeparator})\s*((?:\w+\s*\:\s*)?#{QuotedFragment})/o).flatten
```
Only the first line of this test I would consider to be an integration test.  The rest makes assumptions about the Variable class and methods which we will want to refactor in the future.  This is especially true for the Variable class where it parses expressions when render is called.