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
```