test: added stdlib test

Author: butuzov

test/testdata/configs/ireturn_stdlib_reject.yml

@@ -0,0 +1,4 @@
+linters-settings:
+  ireturn:
+    reject:
+    - stdlib

test/testdata/ireturn_reject_stdlib.go

@@ -7,7 +7,22 @@ import (
 	"io"
 )
 
-func NewAllowDoer() io.Writer {
+func NewWriter() io.Writer { // ERROR `NewWriter returns interface \(io.Writer\)`
 	var buf bytes.Buffer
 	return &buf
 }
+
+func TestError() error {
+	return nil
+}
+
+type Foo interface {
+	Foo()
+}
+type foo int
+
+func (f foo) Foo() {}
+
+func NewFoo() Foo {
+	return foo(1)
+}