Test array filters added in Shopify/Liquid v5.7.2

[jg-rp]

This PR adds test cases for the following filters as they appear in version 5.7.2 of Shopify/Liquid.

• reject
• has
• find
• find_index

Note that these filters do not currently support arguments for testing/selecting child properties at arbitrary depths. See Shopify/liquid#1906.

Observations

Some observations that hold true for all array filters (as of v5.7.2).

• The second argument, target_value, is optional and it does not need to be a string, contrary to what the doc comments say.
• Explicitly testing for null/nil/None is not supported. For example, a | has: 'foo' , a | has: 'foo', nil and a | has: 'foo', nosuchthing are equivalent. Shopify/Liquid does not distinguish between the default of nil and an explicit nil/null.
• Inputs are intended to be arrays of objects/hashes. If the input is an object/hash or primitive value, it gets coerced to a single element array with the input value being the only item.
• In Ruby, subscripting a string with another string will return a substring if the first string contains the second. This leads to {{ 'foo' | has: 'oo' }} and {{ 'foo,bar' | split: ',' | has: 'oo' }} being true, and some odd results when filtering arrays of string values, {{ 'x,y,cat' | split: ',' | reject: 'c' | join: ', ' }}. Shopify/Liquid does include tests covering this behaviour.
• If the input array contains a nil/null/None before a successful match, the result is nil/null/None. {{ a | has: 'z' }} renders nothing when a is equal to ["x", nil, "z"]. This was the case with the where filter before version 5.7.
• An exception is raised if the input array contains an integer and target_value is a string.

See also harttle/liquidjs#799