[flori/json] Use the compiled extension in test

https://github.com/flori/json/commit/148afef84c

test/json/json_generator_test.rb

@@ -243,7 +243,7 @@ EOT
 
   def test_gc
     if respond_to?(:assert_in_out_err) && !(RUBY_PLATFORM =~ /java/)
-      assert_in_out_err(%w[-rjson], <<-EOS, [], [])
+      assert_in_out_err(%w[-rjson -Ilib -Iext], <<-EOS, [], [])
         bignum_too_long_to_embed_as_string = 1234567890123456789012345
         expect = bignum_too_long_to_embed_as_string.to_s
         GC.stress = true