Add tests and NEWS [Feature #18008]

NEWS.md

@@ -96,6 +96,8 @@ Outstanding ones only.
       You need to use a Hash literal to set a Hash to a first member.
       [[Feature #16806]]
 
+    * StructClass#keyword_init? is added [[Feature #18008]]
+
 * Queue
 
     * Queue#initialize now accepts an Enumerable of initial values.

test/ruby/test_struct.rb

@@ -138,6 +138,14 @@ module TestStruct
     assert_equal(3, struct.new(a: 1, b: 2).c)
   end
 
+  def test_struct_keyword_init_p
+    struct = @Struct.new(:a, :b, keyword_init: true)
+    assert_equal(true, struct.keyword_init?)
+
+    struct = @Struct.new(:a, :b, keyword_init: false)
+    assert_equal(false, struct.keyword_init?)
+  end
+
   def test_initialize
     klass = @Struct.new(:a)
     assert_raise(ArgumentError) { klass.new(1, 2) }