Test Ruby3's probably popular syntax elements

See https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/,
but excluding:

- "in is changed …" because that's behaviour
- "Find pattern …" because it's "experimental"
- "Hash#except …" because the test runner doesn't care
   from where the student's code loads this method.

Author: katrinleinweber

test/fixtures/pass_ruby_3_syntax/input/ruby_3_syntax.rb

@@ -0,0 +1,10 @@
+# See section "Other Notable New Features"
+# in https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/
+class Ruby3Syntax
+  def self.rightward_assign
+    'is fun' => exercism
+    return exercism
+  end
+
+  def self.endless_methods = 'are fun'
+end

test/fixtures/pass_ruby_3_syntax/input/ruby_3_syntax_test.rb

@@ -0,0 +1,14 @@
+require 'minitest/autorun'
+require_relative 'ruby_3_syntax'
+
+# minimal set of:
+# Common test data version: 1.6.0 42b9d45
+class Ruby3SyntaxTest < Minitest::Test
+  def test_rightward_assign
+    assert_equal Ruby3Syntax.rightward_assign, 'is fun'
+  end
+
+  def test_endless_method_def
+    assert_equal Ruby3Syntax.endless_methods, 'are fun'
+  end
+end

test/test_runner_test.rb

@@ -29,6 +29,29 @@ def test_pass
     )
   end
 
+  def test_pass_ruby_3
+    assert_fixture(
+      :pass_ruby_3_syntax,
+      {
+        version: 2,
+        status: :pass,
+        message: nil,
+        tests: [
+          {
+            name: "Rightward assign",
+            status: :pass,
+            test_code: %(assert_equal Ruby3Syntax.rightward_assign, 'is fun')
+          },
+          {
+            name: "Endless method def",
+            status: :pass,
+            test_code: %(assert_equal Ruby3Syntax.endless_methods, 'are fun')
+          }
+        ]
+      }
+    )
+  end
+
   def test_fail
     assert_fixture(
       :fail, {