Replace ostruct dependency with RSpec double

There's no need to introduce the ostruct stdlib gem just for this single
object while testing.

This removes a warning about including a stdlib gem without explicitly
depending on it.

spec/lib/pg_search/features/trigram_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "spec_helper"
-require "ostruct"
 
 # standard:disable RSpec/MultipleMemoizedHelpers, RSpec/NestedGroups
 describe PgSearch::Features::Trigram do
@@ -16,7 +15,8 @@
     ]
   }
   let(:normalizer) { PgSearch::Normalizer.new(config) }
-  let(:config) { OpenStruct.new(ignore: []) }
+  let(:config) { instance_double(PgSearch::Configuration, ignore: ignore) }
+  let(:ignore) { [] }
 
   let(:coalesced_columns) do
     <<~SQL.squish
@@ -35,7 +35,6 @@
 
   describe "conditions" do
     it "escapes the search document and query" do
-      config.ignore = []
       expect(feature.conditions.to_sql).to eq("('#{query}' % (#{coalesced_columns}))")
     end
 
@@ -45,14 +44,14 @@
       end
 
       it 'uses the "<%" operator when searching by word_similarity' do
-        config.ignore = []
         expect(feature.conditions.to_sql).to eq("('#{query}' <% (#{coalesced_columns}))")
       end
     end
 
     context "when ignoring accents" do
+      let(:ignore) { [:accents] }
+
       it "escapes the search document and query, but not the accent function" do
-        config.ignore = [:accents]
         expect(feature.conditions.to_sql).to eq("(unaccent('#{query}') % (unaccent(#{coalesced_columns})))")
       end
     end