Fix a spec failure

http://ci.rvm.jp/logfiles/brlog.trunk-test.20210216-182358
```
1)
$LOAD_PATH.resolve_feature_path raises LoadError if feature cannot be found FAILED
Expected LoadError but no exception was raised (nil was returned)
/tmp/ruby/v3/src/trunk-test/spec/ruby/language/predefined_spec.rb:1275:in `block (3 levels) in <top (required)>'
/tmp/ruby/v3/src/trunk-test/spec/ruby/language/predefined_spec.rb:1259:in `block in <top (required)>'
/tmp/ruby/v3/src/trunk-test/spec/ruby/language/predefined_spec.rb:1258:in `<top (required)>'
```

spec/ruby/language/predefined_spec.rb

@@ -1271,8 +1271,16 @@ ruby_version_is "2.7" do
       path.should.end_with?("/etc.#{RbConfig::CONFIG['DLEXT']}")
     end
 
-    it "raises LoadError if feature cannot be found" do
-      -> { $LOAD_PATH.resolve_feature_path('noop') }.should raise_error(LoadError)
+    ruby_version_is "2.7"..."3.1" do
+      it "raises LoadError if feature cannot be found" do
+        -> { $LOAD_PATH.resolve_feature_path('noop') }.should raise_error(LoadError)
+      end
+    end
+
+    ruby_version_is "3.1" do
+      it "return nil if feature cannot be found" do
+        $LOAD_PATH.resolve_feature_path('noop').should be_nil
+      end
     end
   end
 end