Add test and sig for rb_ary_push

Author: sue445

testdata/dummy/sig/dummy/tests.rbs

@@ -28,5 +28,6 @@ module Dummy
     def self.rb_eval_string_protect: (string str) -> [untyped, Integer]
     def self.rb_ary_new: () -> []
     def self.rb_ary_new_capa: (Integer capa) -> []
+    def self.rb_ary_push: ([] ary, untyped elem) -> []
   end
 end

testdata/dummy/spec/dummy/tests_spec.rb

@@ -209,4 +209,11 @@
       expect(Dummy::Tests.rb_ary_new_capa(1)).to eq []
     end
   end
+
+  describe ".rb_ary_push" do
+    it "works" do
+      ret = Dummy::Tests.rb_ary_push([1], 2)
+      expect(ret).to eq [1, 2]
+    end
+  end
 end