Test against liquid-spec [skip ci]

[jg-rp]

Test against https://github.com/Shopify/liquid-spec.

It's quite hard to consume that test suite for non-Ruby projects and there's a lot of behaviour that we would not want to copy.

In it's current form:

• We're only looking at specs/basics and specs/ruby_liquid. We're not trying to mimic Shopify stores and we're not interested in Shopify/liquid in "lax" mode.
• We skip any test case that uses the !binary tag in it's data.
• We skip any test case that has expected_pattern instead of expected (I'm trying to match the Golden Liquid schema)
• We skip any test case that has an unhashable mapping key or recursive data.

Further notes:

• Quite a few test cases are asserting a Ruby-style dict/hash/mapping string representation using => instead of Python and JS-style ":". We don't want to copy this.
• ...