Added version guard for backtrace printing feature of psych to ruby/spec.

spec/ruby/library/yaml/to_yaml_spec.rb

@@ -72,8 +72,16 @@ describe "Object#to_yaml" do
     true_klass.to_yaml.should match_yaml("--- true\n")
   end
 
-  it "returns the YAML representation of a Error object" do
-    StandardError.new("foobar").to_yaml.should match_yaml("--- !ruby/exception:StandardError\nmessage: foobar\nbacktrace: \n")
+  ruby_version_is ""..."2.7" do
+    it "returns the YAML representation of a Error object" do
+      StandardError.new("foobar").to_yaml.should match_yaml("--- !ruby/exception:StandardError\nmessage: foobar\n")
+    end
+  end
+
+  ruby_version_is "2.7" do
+    it "returns the YAML representation of a Error object" do
+      StandardError.new("foobar").to_yaml.should match_yaml("--- !ruby/exception:StandardError\nmessage: foobar\nbacktrace: \n")
+    end
   end
 
   it "returns the YAML representation for Range objects" do